Eeyore wrote:

At only 4 years I'm very doubtful about any ideas relating to corroded
valve bases unless you stored it in a basement for ages.

Of course, valve swapping should sort out any possibilities there.
Remember to only swap one at a time.

Agreeing with this advice is a section on troubleshooting problems in a valve amp manual I translated for a German manufacturer last week:

"Noise, crackling or similar undesirable noises can be caused by residual gas or other residues in the tubes. This kind of noise does not often reach a level that could be considered annoying, although the nearer to the "front" the particular tube is, the more it will tend to make itself felt. In the V 80, this would be tube V 6. As it is rare for both systems in a double triode to be equally affected, the noise levels in each stereo channel will be different. If you experience noise that is louder on one channel than on the other, the first thing to do is to change the V 6 tube (ECC 81/6072)."

Stephen