Now, I have heard a vehicle make a lot of different sounds-most of which have indicated a need to go to the shop- but HUMMING? It happens when I turn off the engine, usually after a very short drive. Say, about a mile or so. It is a Chevy Silverado half-ton pickup, if that helps. I don't remember the year, perhaps it is 4-5 years old? I bought it used last year with 50,ooo miles on it. Any ideas about what could be causing the humming? Mind, it hasn't broken out ito SONG yet, but even so it is really, really DIFFFERENT!