Ok, I'm gonna go ahead and stick my neck out here and say I searched for any topics already out there and couldn't find any, so if this has already been covered I apologize.

I've been getting a humming noise coming from behind the radio in the dash after I start my truck recently. It sounds like a pump of some kind, and it only last for about 3 seconds. So far I've only noticed it when its been cold outside, and its not a very consistent noise, sometimes it happens, sometimes is doesn't. The difference from some of the other posters is that mine happens whether the truck is cold, or was just shut off 30 sec. ago. Not too worried about it since it seems to go away quickly, and I'm sure I'd never be able to recreate it for the guys at the service department. Even so anyone have any ideas on this one? Thanks as always.