Well, I took it to an indy down the street(not BMW specific, but seem to know his stuff). He took it for a "spirited" drive(as per my request) and immediately thought it was a rear shock mount.
We got it on the lift, and to our surprise, everything was on point.
The shocks/mounts were fine and the springs are basically brand new (replaced in August 2012).
He poked around, checked for play in the diff(there's a slow leak, checked levels, everything's fine), violently shook the rear wheels... nothing. He popped the trunk, started pressing on **** trying to get the sound to replicate... nothing. He removed the tools from the trunk mounted toolbox and then we went for a drive.
The noise magically disappeared! He thinks it was the tools in the box, I seriously doubt it as the noise was too loud to be that.
Anyways, I'm pretty relieved although very confused. The shop tech only charged me a round of coffees for the visit - pretty cool guy.
Would the noise just stop if the subframe mounts were cracked/torn? I'd think it would be a constant noise.
I've booked an appointment with the subframe specialists(RMP Motorsports) to have my diff oil replaced/flushed. They'll be inspecting the subframe at that point. I'm not quite due for a flush, but I figure $80 for the diff oil change and an inspection is well worth it. They typically charge $50 for the inspection alone.
I'll post updates on Friday after the appointment.
But do you think the noise would disappear, even temporarily, if the subframe was cracked??