Thanks.. that's a good idea about the A/C. Forgot that it comes on during defrost. I'll give that a quick shot.
The banging is actually during "driving" more than "acceleration". I wasn't able to go over any especially bumpy or smooth roads to find out, but the banging carried on any time the car was in motion. Didn't seem to get worse or better under braking, acceleration, or over bumps.
I had originally thought of subframe problems, but everything I'm hearing says the 380SL was the one with subframe cracking problems. If anything, maybe it could be shocks/subframe mounts? I still think the most likely option is a banging exhaust pipe.
Owner claims the car didn't do that the day before, but his wife drove the car the night before I got there to look at it and she reported the noise to him just before I got there. He seemed genuinely upset that she didn't tell him about it earlier so he'd have enough time to look at it before I got there.
Not sure if that's believable or not, but I'm assuming worst-case scenario that I have to replace one part of the exhaust system (ie. muffler).
'83 300SD 335,000km (207k) mi
'87 560SL 163,000km (101k mi)
'86 300SDL 356,000km (220k mi)
'92 500SEL 250,000km (155k mi)
'90 300SL 140,000km (87k mi)
'01 S430 260,000km (161k mi)
'03 SL500 167,000km (104k mi)
'07 GL320 CDI 4MATIC 289,000km (162k mi)
'07 S550 4MATIC 170,000km (105k mi)