Weird Sound

[Maxil]

Yesterday as I am leaving work, I hop in my truck, and with the door still partly open I crank it up. I noticed right off that there was an...extra sound that normally isn't there. Kind of a medium pitch, it was running steady, not like a loose belt does. I foolishly shut it off for a second and when I cranked it up, it was gone. The noise was loud enough to notice with the doors closed but wasn't super loud or anything, also when I turned the truck off, I could hear whatever it was turning come to a slow stop. It would almost be mistaken for a normal noise.

Does this by chance ring a bell to anybody or do I need to wait until next time to track it down?

Thanks a bunch,
Maxil