SET amp - diagnosing crackling static noise

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I have an older model Antique Sound Lab's MG-SI15DT int amp that has slowly been developing a worrisome noise from the right channel. And before I describe the problem fully, let me just state that I have done enough left-right swaps and tube swaps to determine that the problem is isolated to the amp but is not directly caused by the tubes. I have also checked the tube bias settings.

What I hear is that when playing moderately loud to load (as loud as I can with 5 watts/channel through my Omega 3Rs), I sometimes hear crackling static noise from the right channel. The noise seems to be somewhat dependent on the program material. Some music (particularly female Soprano vocals) can easily trigger this condition. Other music with a lot of mid-treble content can also trigger the same condition. The sound is not unlike the crackling static you might get if you were to tweak an interconnect that has bad contacts. I am starting to suspect a failure of one of the resistors or capacitors in the circuit. How does one go about diagnosing a problem like this?

The left channel always plays clean.

JC