Hi David- as another possibility for your to consider in chasing this squeaking noise down.....
On the E320 Coupe, I had a squeak that was heard faintly in the cabin in circumstances nearly identical to yours. Naturally, I would be able to hear it and no one else would (techs or passengers).
Noise was occurring due to a semi-loose fender and bumper= problem rectified by re-torquing mounting bolts.
Prior to sourcing the culprit(s), there was an extensive search and discovery process that included replacing front windscreen together with multiple occasions SF Factory Authorized Service exhausted inordinate amounts of time reviewing cabin fittings, chair mounts, dash, interior/underneath dash technics, floorboard, engine compartment, suspension, vents, and more misc. "things". It was a real witch hunt where the source of the problem was not immediately proximate to where it manifested itself (since energy can travel through metal and plastic in any/every direction).
I know how irritating this is, so much so, that I was going to give the car away if that rock solid solitude of the cabin would not be restored.
Good luck and hope this helps.