I got a SF champ recently. It plays and sounds great but there is this kind of sound - I'm not sure how to describe it. It's not in the audio path and its still there when the amp volume is down, and it doesn't appear to becoming through the speaker. Its like once of the tubes is humming internally. It's a faint sound but enough to annoy the help out of me - I'm not playing the amp loud, maybe one 4. I took a sharpie (its non conductive) and kind of probe the tubes, seeing if there were any issues, and pushing on the base of the 5Y3 seems to change it a bit. The 6v6 and 5Y3 are old, so I was thinking maybe it's one of them. Has anyone experienced anything like this?