Vomit

Over the past week, my '02 C32 has started making a strange thumping noise from behind the steering wheel right after the car is started. It makes the noise once every 5 seconds for about a minute, and is not dependent on movement of the steering wheel and is not related to the HVAC system (at least I don't think so, as it does it whether the fan is off or on).

Any thoughts? It is covered by warranty, but I thought I might shave a couple of days off of my service visit by telling the MBZ brain donor "tech" what the problem is before they go tearing my car apart.

Thx for any info.