When I drive my intrepid, after about an hour or two of sitting off, when i accellerate, there is a slight hum coming from...idk where but as soon as i hit about 30 it stops. also in the warmer months when using the a/c it does it too. The previous owner told me of this when i purchased it. she said that it started when they had the tranny rebuilt. Anyone have an idea of what it is and how to stop it?