strange oscillating thumping sound at speeds of 70mph or so

[stevehifi]

Hi Guys,

I'm running 19x9.5 with 265/35/19 tires on Model 3P and I noticed at speeds of 70mph+ I get a strange oscillating thumping sound emanating from the car with a little vibration under my seat. It seems to happen on and off when I'm driving. I could drive a couple miles and then it slowly fades away .. and then it comes back and intensity increases - so it doesn't happen all the time.

Hard to localize but it seems to be coming from the rear of the car.

Not sure what it could be (wheel imbalance, something rubbing on wheel, undertray loose and wind making it flap, etc) but just curious if anyone has encountered something
similar before.

Thanks,
Steve

 

[tomtea]

with the m3p, did you take off the front rotor retaining bolt before installing the new wheels?

 

[stevehifi]

I have the MPP front bbk which doesn’t use a retaining bolt. For the rear I also use the MPP rear bbk I did use the stock retaining bolt since the MPP rotor has a hole available. I thought it would be better to keep the rotor secure but perhaps it’s contributing to uneven balance?

 

[tomtea]

try it?

 

[Vaillant]

Do your tires have acoustic foam? I had issues with the foam causing vibrations at 70+ mph. Started suddenly one drive. Didn’t go away until I got new tires.

 

[stevehifi]

update, with both Uber 20s and 19s the issue is the same. I just re-aligned the car and it's still present.

removed the retaining bolt on the rear rotors and no change.

I'm going to try and see if it's the rear hatch since I know there's a know problem with the stops for the hatch which need to be adjusted to prevent
the hatch from rattling.

If anyone has any other ideas i'm open to suggestions.

 

[XPsionic]

Just throwing out ideas based on what you said and what you did

Wheel bearings, drive shaft, motors, loose undertray, garbage bag stuck somewhere that's flapping around

 

[Clover]

If the problem is present with different wheels and tires and the alignment is good, I'd check the wheel bearings too. Could have one starting to go and it would be worst at high speeds most likely. If it is not the wheel bearing, you may need to start checking the driveline which would not be good news. You could check if you have any plastic panels loose, but I would expect you to hear that before 70.