By on March 21, 2017

Picoscope NVH diagnosis tool, Image: Twitter

Doug writes:

I have a 2007 Mercedes-Benz S550. I accidentally bought it in 2014 off eBay (long story) for about trade-in ($13,000), with 150,000 miles on the odomater. In a twist of good luck, it turned out to be a one owner car and using CarFax I was able to see and verify that it had been maintained by the selling dealer right up to a few weeks before its eBay appearance. A call to the dealer confirmed the complete service history. Even the brake pads and rotors were new, and it had a newish and very expensive set of Michelins. Almost three years later, it has been very reliable for my teen daughter and has 180,000 miles. It had a few quirks I have fixed myself (clogged charcoal canister, minor trunk leak caused by a missing rubber plug, sticking power driver seat) and only one real repair (dead stereo amp rebuilt by Becker).

Now that my daughter is off at college and content with a ZipCar, I am driving it and intend to keep doing so. I have noticed a vibration in the center console area of all places, while driving around town. You feel it through your arm resting on the console lid. It’s a deep vibration, if that makes sense — coming from under the car.

You don’t feel it in the seat, nor do you feel it in the steering wheel. You do not feel it at a standstill, just at 20-30 mph. It goes away at higher speeds, and the car is rock solid and smooth as glass at 75 mph. I am stumped as to the cause, and with the age of the car, I don’t want to set a dealership or even an indy shop loose on it without more of an idea of the cause. I was thinking maybe a motor mount, but it seems like I would feel that all the time, and especially at idle, which I don’t. I was wondering about maybe some kind of driveshaft or transmission mount or connection point, but it seems like a fault that would get worse with increasing speed instead of going away. Do you have any ideas?

Sajeev answers:

Dumb luck or no, good on you for finding a cherry W221 S-class. These seem significantly more cost effective than the W220 (don’t take my word for it) so, faint praise aside, fixing this NVH issue isn’t a waste of money.

This problem is likely a driveline issue, either the driveshaft, the transmission and/or differential mounts.  With your email’s signature file in mind, your career likely gives you neither the time nor the knowledge to make this a DIY repair.  Either you pay someone to throw parts at it (good luck) or you find a shop investing the coolest NVH monitoring tools. Time to find such a shop, or even break tradition and take an out-of-warranty S-class back to the dealership.

Your symptoms remind me of when I had (slightly) deformed differential mounts on my 1995 Lincoln Mark VIII, but no way in hell am I making that recommendation. NVH troubleshooting is not for the faint of heart.

I am confident you’ll pay more for a proper diagnosis to pay less overall.

[Image: Picoscope/Twitter]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice. 

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27 Comments on “Piston Slap: NVH Nightmares No More?...”


  • avatar
    notapreppie

    Does the driveshaft have a yoke that mounts up in/near the driveline tunnel? How about a transmission mount?

  • avatar
    Sam Hell Jr

    So if these vibrations had been solely at idle, and went away in neutral or park, and didn’t seem to be affected by load on the alternator, then motor or transmission mounts would in fact be a more likely culprit? Asking for a friend who drives an ’11 tC and posts as Ham Sell Sr. ;)

  • avatar
    Sam Hell Jr

    This guy’s accidental purchases are way more fortuitous than mine. Awesome car, still looks like money.

  • avatar
    smallblock

    My guess is the driveshaft carrier bearing mount. Get under there and inspect it, see if the rubber’s degraded, try to shove it around.
    Cayennes & Touaregs (totally different vehicle, I know) were notorious for this, and the carrier bearing wasn’t available as a separate part. The aftermarket came up with a decent solution. http://epsdriveshaftbearingsupport.com/

  • avatar
    dwbf11

    I don’t know if this is helpful, but there was an almost identical issue that plagued Acura TL’s manufactured in the mid-2000’s, except from 50-60MPH. The TSB called for the installation of a harmonic dampener in the subframe, near the rear motor mount. Perhaps there is a similar issue here.

  • avatar
    gtemnykh

    I will echo others in that I suspect some sort of driveshaft carrier bearing type of thing, or perhaps a transmission mount. Lots of vehicles use these random looking blocks hanging off of brackets that engineers placed just so to dampen harmonic vibrations like that. As mounts wear/age, sometimes that original damper is no longer in the sweet spot. As an example, there are widespread complaints on gen III Monteros of a driveline/engine hum at about 2200rpm, the most cost effective solution that takes care of most of the problem is simply to take off the factory harmonic balancing block, rather than replacing transmission mounts. I think Mitsu even made that a ‘factory fix.’

  • avatar
    DIYer

    Check the driveshaft flex disc couplings, one front and one rear.

    Could be the center carrier bearing, but the flex discs are the more likely culprit at 150K+

  • avatar
    bam210135

    Check the rubber flex disc(s) at the ends of the rear driveshaft and the rear driveshaft carrier bearing. The rubber can crack and cause weird vibrations. I’d recommend trying motor mounts and the trans mount if you don’t see anything wrong with the other pieces. It’s not a bad idea to replace them anyway at 180k.

    • 0 avatar
      55_wrench

      What BAM said.
      Get under there and at least check out the flex disks. I used to wrench on older W123 and W126’s and these were considered up for replacement at about the age and miles of this car.

      If one of these lets go, your driveshaft will reform your floor pan in ways you never thought possible, and could possibly take out your tranny tailshaft housing.

      Look for exposed fibers, dry rotted rubber, that sort of thing.

  • avatar
    MPAVictoria

    My advice is to ignore it until it gets worse. Then it will be much easier to track down.

  • avatar
    ajla

    So is the C216 also relatively “cost effective”?

  • avatar

    I recently saw a newish M235 on the rack. There was a weight placed on the rear subframe, about the size of a wallet. It had no obvious purpose, and wasn’t connected to anything…, and the assembled group wondered. Our Master Mechanic said “when I see one of those it usually has something to do with vibration”.

  • avatar
    Eddie_B

    It makes me irrationally happy to know that somewhere a teenager daily drove an S550 until she outgrew it. On the other hand, it’s going to be real tough to ‘upgrade’ later in life.

  • avatar
    whitworth

    Could be a transmission mount. I tend to think motor mounts you can feel in the steering wheel.

    I just know when I replaced my motor and transmission mounts after 150k miles on my Lexus, it was a night and day difference. You can barely tell the car is running.

  • avatar
    dchturbo

    My bet is tires. Try rotating them and see if the vibration changes/goes away. Cheap, easy, and clear results right away.

  • avatar
    Importamation

    This is my car/query….thanks for the write-up and everyone’s ideas! Keep them coming. I guess it knows we are talking about it, because I drove about 250 miles today for work and I swear I felt less of the vibration than usual. It was also unusually hot today for this time of year….about 88 F. The hottest day for sure since the vibration started 3 months ago. So maybe that’s a clue. I have read there is indeed a rubber center carrier mount/bearing on the W221, so that may be the prime suspect. A hot day reducing the vibration might be related to the rubber parts (maybe the flex disks too) swelling back out instead of shrinking from the cold (we were below freezing last week). It’s about due for new tires and I have a set of Pirelli P7’s off eBay that arrived today, so we will see if that makes any difference as well.

    • 0 avatar
      Tosh

      But you said “You do not feel it at a standstill, just at 20-30 mph” so htf is it anything to do with tires (or driveshaft)? Please clarify.

      • 0 avatar
        Importamation

        I agree it is odd……but those are the speeds where it is primarily felt. It goes away at higher speeds. I don’t have experience with drivetrain vibrations but tire vibrations in my experience certainly can come and go according to speed……a tire/wheel balance problem may be evident only at certain speeds for example.

  • avatar
    pwrwrench

    Yep, driveshaft rubber flex damper(s) are the likely culprit. There is usually one right under the driver’s right elbow if it’s resting on the center console.
    Most Mercedes and BMW RWD/AWD have a multi piece drive shaft. There are usually needle bearings at the shaft joint(s). The car in question is at about the miles where those bearings wear out causing vibrations.
    Find a driveshaft shop that is experienced in Mercedes.
    A-number-1 No-No is to disassemble the driveshaft without marking the relationship between the parts. It’s very difficult and time consuming to get them back together correctly. Most experienced techs will tell you to scrap the drive shaft and buy a new one. BIG $$$.

    • 0 avatar
      DenverMike

      “BIG $$$.”

      You could be down $10,000 while they keep guessing. Get 2nd opinions and research each potential and expensive “fix”.

      Start with the cheap/simple fixes, even if shops recommend the high ticket stuff. Are they going to guarantee the vibration goes away? Hell no! All they see is a boat payment pulling into the shop.

      • 0 avatar
        Importamation

        I agree. I’m not willing to turn it over to anyone yet at this age and mileage. I can live with it to a degree, but I don’t want some serious failure either. I have a tenant that has a two bay garage where he wrenches on Hondas, mostly. I think I’ll ask to put it up on his lift and I’ll inspect these rubber parts myself one Saturday and see what I find.

  • avatar
    DenverMike

    Sometimes you just have to put it up on the rack and give it hell. A bent wheel or flat spot on a tire wont be felt at low speeds, and at high speeds, the wheel velocity hides the defect.

    Ever notice on washboard roads, when going over 50 mph everything smooths out?


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