As said, leaky capacitor, bad contact on sockets and valves
themselves, plus dry joints.

Dry joints have been the commonest cause in my experience. If you use
point to point wiring and ANY older parts (tag strips, sockets), you
are at the mercy of the metal surfaces - these can be oxidised, dirty
or even just plain badly made from metal that won't solder properly.

Since all the above can be found in valve amplifiers of a certain age,
you sometimes need to re-solder all the contacts to eliminate a
stubborn rustling. Could be a hairline crack in a joint, could be
virtually invisible. The rustling is presumably some sort of bad or
intermittent contact in these cases. I've usually cured this by re-
soldering the joints around where the rustling comes - you can
generally localise it by a certain amount of trial and error. Mostly
it was valve bases.