Without reading every post, I can relate my story...had a creaking EXACTLY like what you described...caused by a loose subframe and a ball joint and upper control arm that needed replacement.

The motor mounts were fine, they said, as they handed me the bill for around $1300.00.

In my case, the creaking mostly came from the sub-frame "swimming" around under the car...and yes, it creaked when I got in and sat down...(although I do have a rather intimate--260 pound--relationship with gravity).

__________________
Tom

'87 560 SL