Just a thought - have you checked the transmission exhaust hanger? It has 2 bolts sized 13 mm that often come loose causing it to rattle noticeably at idle, especially with the A/C on. Check that first; they often just need to be snugged up.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280