Strange thumping sound - morning start

[projectasia]

Hi all, 2007 4.8i. When i start in the morning, I hear the CD player do something, then, several seconds later, I hear a thumping sound from under the dash.

I can best describe this as "sneakers in the dryer" sound. ka-thum, ka-thump, about 1 thump per second.

It last for about 5 seconds, then it stops, then the RPMs drop, as if to say the startup /warm-up process is complete.

I never hear it until the car sits overnight on first time start for the day.

I tried listening under the hood, i cannot hear it. Also tried with my ear next to each front tire, I cannot hear it. I can only hear it inside with windows up. I can actually feel the car shaking slightly.

Any ideas what it is? Or i should just turn on the radio in the morning? thanks in advance.