My S420 periodically has a clack like that. I did some very close listening and it turned out to be a cat-converter that was rattling. It might be worth a second look/listen.

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles