Amplifier problem

[Musepro]

I own a pair of powered speakers and have started having problems
with one. After it's been turned on for a while, a white noise kind of sound
starts coming from it until I turn it off and back on again. After another while, about 20 minutes or so, it starts again!

I completely unplugged the speakers input and tested it. Again, the noise came out after a few minutes, so it's a problem with the built in amp. I took it apart and didn't see external damage to the components or any bad solder joints.

Is there anything I should look for or test? Thanks!

jt burke