Rapid ticking sound
 

Could any one please hazard a guess what might be making a rapid ticking sound audible through the engine noise but not coming from the engine and not coming from anything that is connedted to the engine through any of the belts. The frequency does not coincide with the engine revs but its pitch gets slightly lowder under pressure. This is on S500 done only 14000 miles in 2 years.
To me it sounds like a rotating component, may be a pump of some kind possibly located somewhere under the engine. The man at my Merc, service has not been able to diagnose it but is sure that it is not the injectors.