A bad ground may be the case. If not, take a look and see where the main power cable is located. What happens is that the power coming from the alternator (AC power) can leak into signal carrying cables that run parallel to it (has to do with the electromagnetic feild given off). As the engine revs, the AC frequency from the alternator increases. Chances are you hear it in your tweeters and midrange before you hear it in the subs because it is at a fairly high frequency. The person before you may have installed an aftermarket stereo and put it back together half ass. To fix, run the power cables on one side of the car and the signal carrying cables on the other.