Piston Slap: UR U Joints Be Bangin' Dat Stang?

Mark writes:

Sajeev–

Here’s a weird one to test your skills of remote diagnosis. Fire up your Magic 8 Ball for this one.

Car: 2012 Mustang V6, manual gearbox, performance package (currently running some crappy General AS Max-03 tires), 33,000 miles. I’m the only owner; special ordered to get it just the way I want. Mods limited to a Shaftmasters one piece drive shaft ( scared into it by a few YouTube videos), a Draw-Tite receiver hitch to pull my one bike motorcycle trailer, and an 87 octane tune uploaded via SCT (on your recommendation). The car will go into a nearby Ford dealer for a look at this issue under warranty this coming Friday, but based on the “quality” of some of the work I’ve had done over the years, I thought it would help to give the techs some clues.

Problem: For the past few days, I’ve heard irregular “bangs” or “clunks” from under the car.

The noise is heard, not felt, and I’m unable to pinpoint the corner of the car from where the noise emanates. It doesn’t seem to be speed related: the noise can happen at a walking pace on up to about 40 mph, and the frequency doesn’t vary with speed; the noises can be 2 seconds apart or 30 seconds apart. I think the noise continues at higher speed, it’s just drowned out by wind/road noise. Some bumps set off the noise, some don’t…can’t seem to figure out any common threads on the bump-noise relationship. The one thing I have noticed: no noise if the brakes are applied. Even a very light drag of the brakes silences any noise.

My Own Research: I’ve had the car jacked up in the garage and I don’t see any obvious issues. The exhaust system seems to be well secured, as do the anti-roll bars. No obvious hanging parts. The brake pads have plenty of meat. No leaks from the shocks/struts. The receiver hitch is not loose.

This car has had a few chassis issues: a new steering box around 15k and a new rear anti-roll bar around 30k. I blame the poor condition of roads here in Illinois for those failures, along with the lack of give from the low profile 40-series tires.

Some digging in Mustang forums failed to uncover a smoking gun.

Bonus Question: When these crappy Generals give up the ghost, do you see a major problem with switching to 45-series tires in an effort to gain a little more bump compliance?

Thanks, Sajeev!

Sajeev answers:

I’ll answer the easier (bonus) question first: sure, no problem and it might help a little. But wanting bump compliance from 19″ wheels on a car is like expecting an honest answer from a politician. Both are laughable: try minus sizing via 17″ wheels from an older S197 (as they get dumped on craigslist for dirt cheap) over the performance package’s upgraded front stoppers. If not, maybe 18″. If not, give up be awesome and get a Grand Marquis as a second car.

That was easy. Now, I suspect your first question shall have no solution when a mechanic starts prying/wiggling suspension things to test for play.

I commend you on the modifications, but…there’s always a but. My gut thinks the new U joints on that new (and necessary) upgraded driveshaft are the problem. Can you (gently, with millimeters of throttle input) load/unload the drive line while the tires are losing/gaining traction? Like maybe on a bumpy/slick road at highway speeds. If the thud/clunk comes back, it’s the U joints.

If not? Maybe it’s a shock/strut mount. Or maybe a sway bar mount, but the speeds you mentioned make me think U joints Über Alles.

[Image: Shutterstock user Tossapol]

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.