M104-AMG

iTrader: (8)

I heard a low-pitch rumble that begins at about 25-mph, and can really be heard at 30+ mph. There was not drivetrain vibrations or thuds.

It doesn't matter which gear I'm in, even if I'm in neutral coasting at 25+ MPH.

We placed the FD on the lift, started her up, kept the revs to 2K RPM, and placed it in all gears, while my buddy with an automotive stethoscope listened to diff pinion gear, diff center and sides, and then wheel bearings. FWIW- diff side seals were replaced a week ago, and new diff oil.

He also listened to the tranny.

All were good/quiet.

He could hear the low-pitch rumble and it can be heard on the PPF. We think it could be the driveshaft U-joints, but won't know for sure until we take it off.

What are the symptoms of a bad driveshaft / driveshaft U-joints ?

Could a mis-aligned PPF do the same thing, but be speed dependent ?

Note: the PPF hasn't been touched/removed/disassembled for 3+ years, and I haven't had an off-course excursions.

TIA,
:-) neil

IRPerformance

iTrader: (41)

Pull the driveshaft off and check the u joints. That being said, they don't fail too often.

WJM ROTARIES

Sounds like noisy rear wheel bearings .You wont hear them on hoist because the car weight is not on them

DaleClark

iTrader: (55)

+1 on wheel bearings. The driveshaft is easy to check off-car but REAL hard to check on-car. Off the car the u-joints should flex smoothly with no binding or slop.

FD driveshafts rarely if ever fail. I don't think I've ever heard of one going out. Good thing is with the powerplant frame the trans and diff are locked together so the U-joints have little to no flexing to do, that helps out a lot. When you are flexing and loading the u-joints that puts wear on 'em.

Non-turbo FC driveshafts went out fairly often, and even more so if the car had a broken diff mount so the driveshaft was doing a lot of flexing.

Dale

M104-AMG

iTrader: (8)

We can hear it on a lift @ 30-mph. Stethoscope proved NOT left or right wheel-bearings, or diff (pinion, carrier, and sides, and NOT tranny as well.

Neil

Fritz Flynn

iTrader: (263)

Yep wheel bearing definitely make the hum sound but usually start out with one going bad and only heard when taking a corner and not when going in a straight line.

Neil,
I have plenty of good drive shafts so if you need one please let me know.

Never had a DS go bad but certainly sold a lot to folks with bad u joints.

Have you had the DS off the car recently?