Thunking after subframe reinforcement and new subframe bushings?

[sammyf]

So the car went through an pretty extensive refresh (Engine Mounts, Tranny Mounts, Guibo, AND now CSB as well which are ALL BMW). I noticed it mostly in high gear on the freeway; sport mode on, in 6th gear and would punch it to pass and THUNK. But my buddy says he notices the same thunk in every gear. We did retorque sub-frame bushings... I'll check the diff bushings as well.

[old///MFanatic]

Now, when I am in the car (especially in sport mode) when I punch the throttle in a higher gear (6spd manual), I get a pretty noticeable "thunk" coming from the back. We actually had the car on the lift in the air and are able to hear it inside the car, but when we are looking around outside underneath, we can NOT hear/see anything.

That suggests it’s leaning towards something bolted directly to the chassis. Cause noise through the chassis.

So we replaced the center bearing with factory part...UNFORTUNATELY, the thunk is still there. Any other ideas? Again it only happens during initial acceleration.

Thought you said guibo before, I directly asked and questioned. You also said it’s in high gear. Now seems saying it’s on initial acceleration. Initial acceleration sounds like the subframe bushings. It takes up the initial torque impact, OEM bushings are designed just to alleviate a lot of this. Poly move less and by design remove the initial impact factory design buffers. So it will cause a thudded impact on the four bolts/studs that hold the subframe. So it could certainly be the poly subframe mounts. I’m not saying that’s what it is without examining but certainly could be. I’ve seen rear diff bolts torqued wrong and also cause thuds, so there’s more things to check. Are the new motor and trans mounts OEM or poly?

[sammyf]

Here's the bushing setup in the rear. Diff bushings are all BMW (BRAND NEW NOSE and ORIGINAL REARS). We have video evidence that the DIFF is NOT moving under load.

[Tbonem3]

It's really bad? It's not just the natural clunkiness of the driveline?

Poly bushes will exacerbate noises.

[sammyf]

So we replaced the center bearing with factory part...UNFORTUNATELY, the thunk is still there. Any other ideas? Again it only happens during initial acceleration.

[jet_dogg]

Will never understand why people exchange US currency for polyurethane bushings.

[SteelGreyM]

Oh no I just installed powerflex poly diff bushings. Not liking what I’m reading.

[old///MFanatic]

lvm3sm46 We took a camera and it looks like there might be a crack around flex disc. Will replace and report back:

You sure that’s the flex disc? Almost looks more like top side of a motor mount.

[Tbonem3]

Pre-load the CSB

[Arith2]

Solid suffrage bushings plus ploy diff bushings aren't for the faint of heart. It stresses me out to hear my diff chatter sometimes. I also have a lightweight flywheel so that really changes things. You'd be amazed at how much a stock flywheel actually dampens.

[sammyf]

lvm3sm46 We took a camera and it looks like there might be a crack around flex disc. Will replace and report back:

[sammyf]

Its a brand new front BMW diff bushing but the rears are original as they didn't look bad on inspection. Car is 2002 with 112k miles.

[lvm3sm46]

Drive shaft support and bearing. Replace that. It solved my clunk

[sapote]

when I punch the throttle in a higher gear (6spd manual),

Higher gear as 5th or 6th? Or 1st?

I asked because the rear end is under higher load with 1st or 2nd than 5th or 6th.

[maw1124]

+1 on non-OE diff bushings... seems like almost 100% when people use nonOE diff bushings, yes?

maw