You should swap the tubes between the left and right channel; if the sound moves, then it is in the tubes and not the capacitors. If it doesn't move, I would suspect an electrolytic capacitor. If it moves, then swap the small signal tubes to determine if it is those tubes or the power tubes. Keep doing the swapping until you figure out which specific tube is at fault.
I've had several small signal tube (6sn7) that made noise (popping and whistling noises) on power down, but were otherwise quiet.
I've had several small signal tube (6sn7) that made noise (popping and whistling noises) on power down, but were otherwise quiet.