KEL84 noise in left channel

[YummyFuzz]

Hello WD forum,

I want to see if anyone had some suggestions on some noise coming from my KEL84 amp if I describe the symptoms.

Setup: KEL84 powered-on with speakers connected, but no audio inputs.

- A moderate volume intermittent popping/rustling comes from the left channel only for about 10 minutes after powering on from cold.
- If left on, the sound does not seem to reoccur when the amp is warm/hot.
- The sound stays at the same level, regardless of the position of the volume knob, or the input/mute selection.
- The sound remains in the left channel even when I swap the tubes one-by-one between left and right (i.e. It doesn't follow a tube).
- It's not the speakers (if I swap the speaker wires L2R and R2L it comes out the right speaker).

If there is an input audio signal playing (at low volume) during this time, the popping rustling can be heard just under the audio. The audio does not cut out or reduced in volume when this noise happens.

So I'm guessing it is one of the electrolytic capacitors on the left channel of the amp, which I can try replacing.
Does this seem right, or are there other tests or components I should try?

(This is the KEL84 that I found at the recycling centre so I'm not sure where or how it may have been stored in the past.)