[Boatanchors] Here's a vintage stumper..

[Flowertime01 at wmconnect.com]

Stephen Tetork,

Noisy caps.

Stephen the old caps will do this to you. The R390 guys call it popcorn noise.
It just depends on how the cap feels. Under transmit you may stress it and 
cause it
to calm down a bit. You really cannot test a cap for the problem.

Check the tubes, inspect the solder joints for cold solder and corrosion.
If the problem still persist, consider changing the cap or caps if they are 
use din pairs.

Roger KC6TRU