Thunking after subframe reinforcement and new subframe bushings?

[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]

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.