Since Omnipressor has stopped working all of a sudden (see other post), I thought I'd try the O-Pressor in UltraChannel.
As soon as I inserted it, I got a horrible loud squeal. Disabling Compressor, O-Pressor did nothing, but disabling the EQ worked. I was left with a quiet residual 12kHz squeal.
I re-enabled the EQ and the loud tone returned. I disabled each band one at a time and found that the High Shelf was causing the loud squeal.
I opened a sprectrum analyzer and screencap software and here is the result: