Weird Vibrato oscillation in a Princeton Reverb Clone build

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Hey Gang,
Still working on this Princeton Clone I built. I am getting a weird harmonic pulsing in the vibrato when it is turned up. It sounds like when 2 strings on an electric guitar are slightly out of tune and it creates that pulsing oscillation. Any ideas on what I should check? It is the bias type vibrato circuit.

Thanks for your help,
Tim