I have a Super Redhead that has started making an intermittent hum. This is not a cabinet buzz/rattle but a hum that changes volume when you tap the chassis. Originally, I believed it to be a ground issue because sometimes just touching one of the jacks would change the volume of the hum. Tapping on the chassis would always produce a result. Each tap would change the volume of the hum. Sometimes I could tap it away but often I couldn't. Also, this hum would sometimes announce itself with a loud crackle through the speakers. Ocassionally, the chassis tapping routine would elicit a crackle but usually not. So I took it to the shop. They re-flowed a bunch of ground connections and replaced a couple of bad plate load resistors. They ran it and pronounced it fixed. Well... it ain't fixed. It's much better but not fixed. Touching the jacks no longer has any effect but the hum still shows up. However, now it is at least predictable and appears to be initiated by chassis vibration. It can be brought on by the old chassis tap or by a loud note. It's not quite as loud as it was but it is still very annoying. The good news is that now, cycling the power will remove the noise until the next loud note. Anybody have any suggestions for a cause and solution for this issue?