I just recently noticed a nasty hum coming from my engine compartment. I think that it only happens when I am not on the gas, so like at stop lights. I think it also only happens when the car is cold, and not warmed up. The car is approaching 40k miles, and this is my first problem with it. Now I do not know much about cars, but I can tell you that it is loud. You can hear it inside the car, and outside from about 50 feet away as well. I haven't been able to play with stuff enough to realize if it only happens when the ac/heat is on or what, but I know that I have always had the defroster/ heat on the last week. I was wondering if anyone had any ideas on what may be the cause or not...or anything I can do to pinpoint the problem?