Trends TA-10 Issue

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I have 3 of the Trends TA-10 amplifiers which I upgraded with the Audio-Magus Tweakit 2 kits. I have had them for a couple of years now.

Once in a while (maybe every couple days that I hear), they will produce a pulsing tone (maybe 300 Hz or so) for a few seconds. All 3 of them have this issue.

Any ideas about what might be going on?
 
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