Hey guys, I recently picked up a used Hartke 3500 head to run through my 4x10 cabinet. I'm not a technical geek, but sometimes the amp runs just fine, and then at other times, I turn it on, and get a loud hissing noise. You can hear a little bit of the bass, but it gets drowned out by the hissing. I'm thinking there might be a loose connection somewhere inside the amp. Again, I'm not sure, so that's why I'm asking. Anyone have any idea what might be wrong here? Thanks in advance!