Piston Slap: NVH Nightmares No More?

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: Shutterstock user Joyseulay]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.