I'm having a look at an AC15CC a friend dropped off to me. After just a few minutes warming up, it starts to make weird noises: hisses, squeals, whistles, you name it. The one thing I noticed is that just about anything tapped inside the amp causes at least a small amount of audio artifact to the output. But, I believe the issue is somewhere around the power tubes. If I tap them, or tap the wires going to their sockets, I can make a lot more weird noises happen. I didn't think both power tubes could be simultaneously bad, and I only have one spare EL84 handy, so I replaced one of the tubes. It still makes the noises. I think the replacement tube is more behaved, but it's very hard to tell since tapping anything in that area seems to cause an issue the closer I get to the tubes. I tried cleaning the tube sockets and resoldering the connections to the sockets, and the noise is still their. I think I'm back to probably bad tubes, but wondering if anyone else has any suggestions... Thanks.