Hi Eric,
Sounds like a typical RF oscillation in the amp during warmup. Characteristics change with temp, voltages etc. When the amp stabilizes, the oscillation stops.
Typical symptoms include noise, hiss through tweeter, clicking, distortion of music etc, depending on strength of oscillation. I have a SS monoblock amp right now that has a moderate RF oscillation on the negative portion of the signal waveform, gradually diminishing to 0 at 6 watts output.
Hope this helps.