I have a 2008 Chevy Cobalt. When I start it from a cold start, it sounds fine, then, about 15-20 seconds in, there is a loud, short (I don't think it's even a second) grinding noise, then it sounds fine again. The noise is preceded (from start up) by a slight whirring noise that I would never have noticed other than the fact that it disappears right after the grind. The car does not make this noise if it is warm. Any ideas?