This is a long shot..
On my w124 i had a strange rythmic clicking sound which increased in tempo with the cars speed.
It was hard to tell exactly where it came from but it turned out to be the re-settable odometer that was stuck "on tooth". I pushed the 'reset' pin a couple of times and it went away..
I don't even know if the s320 has mechanical odometer, but if it does you can give it a try.
Never criticize someone until you've walked a mile in his shoes. That way, when you do, you're a mile away -and you have his shoes!
'86 300E optically converted to '95 E300 (Sold)
-Blauschwarz on silver
-17" AMG & 15" AMG for winter