After i have been driving my car for a bit, when i come to a stop there is a rumble that i can hear with my window down. Its a deep sound, like a bad bearing might make. Today when i got home, i left the car in park and opened the hood and listened around but the results of my un-tuned ear were inconclusive as to where the noise might be coming from, although it seemed lower down somewhere. I plan to take the main belt off and spin the water pump and alternator to see if their bearings are bad. I hope its not the transmission

Any suggestions on what to inspect?