It’s probably a tube that’s gone microphonic.
First, disconnect all pedals and the guitar, both from the front of the amp and the loop, to eliminate other possible sources of hum and noise, prevent ground loops, etc., from confusing the situation. When you turn the amp volume up with nothing plugged into the amp, does the noise persist?
If there’s no noise, it’s probably something else in your rig. If the noise is still there here’s how to check your tubes:
Very gently tap each tube with something like a chopstick or pencil. Some tapping noise may come through the speakers softly even on a good tube, so you’re listening for a loud tapping noise as though you were tapping the front of a microphone.
The first preamp tube (V1) closest to the input jack on the amp will be a little more sensitive, even if it’s good (it’s the one that raises the volume level the most) but if the tapping noise comes through into the speakers loudly on any tube, it’s probably microphonic. Replace it with a known good tube (just get a new tube if you don’t have any extras on hand), and try the amp again to see if the noise stops.
If that doesn’t work, try removing each tube, one at a time, replacing it with the known good one, and if there’s no difference, reinstall the tube and move on to the next tube. Be patient. It’s really not difficult to do this. Just realize it’s going to take a few minutes, and think of it the way you’d think of changing strings.
You can do the same with the power tubes (and rectifier tube if the amp has one).
Or you can try replacing the entire set; this is a good idea if the tubes have been in the amp a long time, though preamp tubes can stay good for quite a few years (it’s good to have spares, anyway). Power tubes should be changed every 6 months - 2 years depending on how often you run the amp, how loud you play, etc.
They aren’t making tubes like they used to, corners are cut these days, and this stuff happens.
