[HAM] Adjusting preamp trimmer capacitor--pulsing noise?

[Randy Kesten]

Is it normal to get a pulsing/beating noise when the expression pedal is
fully depressed and the AO-28 (1957 C3) preamp trimmer is turned
counterclockwise? The Leslie (122) isn't really overdriving but I can't turn
the trimmer up any more without getting this rapid bap-bap-bap sound. It
goes away if the expression pedal is backed off a bit. Any help will be
greatly appreciated!! Thanks.

Randy