I've got a little old fender frontman ss practice amp, which when not playing is fine, no noise, and I can play without issue, but when I play certain notes the amp makes a static noise. At first I thought it might only happy if I play low notes on the E string, which may indicate an issue with the speaker. But I can play some low notes on the E fine, and when i play some notes on other strings they also cause this static noise, so I've ruled out the speaker as the problem. It seems very random, and can't figure out what the problem is?