This is going to be a completely uninformed post, cause I know almost nothing about bass guitars or amps.

Anyways, I borrowed my friends bass and another friends small practice guitar amp (not high quality and not a bass amp), and Ive been teaching myself to play. The bass is an Ibanez of some source. Every now and then, the amp will start making a constant-pitch humming noise. The noise will sometimes fluctuate if I mess with the guitar, and will even stop if I hit a string, but not always. Today, it happened in a peculiar way. I was playing, and when I hit the lowest string, it would make the normal noise briefly and then start to become extremely distorted. It did this a couple of times and then it started doing the noise thing, but during this time, the higher up strings played normally (until the constant noise came in).

Unplugging the cable on both ends, turning off the amp, then firing it all back up didn't stop the noise.

Does this have anything to do with the two 9 volt batteries in the back of the guitar?

I think my friend had the same problem from time to time, but I don't remember exactly what his problem was. I just remember a couple of times where he would get a bizarre sound and he would mention the 9v batteries. In case this isn't the same problem he had, I don't want to bother him with it. Id rather return it fixed if possible.