OP: That’s probably a leaking AC coupling cap.
The longer you wait the more expensive it might be to fix it.
In tube circuits there’s a high voltage bias on the signal, 100V or more, but the AC component is very small, typically under 10V peak to peak.
There should be a cap before the volume control that removes the high voltage DC so the volume knob and everything after only see the small voltage AC. When the cap breaks down and allows some or all of the DC, the volume knob starts feeling all that voltage, and that’s a bad thing. :)