I just did and I think I know what you mean.
Try it with the 'Brute factor' knob turned down to zero and see if it still does it.
On mine, with the 'Brute Factor' turned up anywhere past 9 o'clock, applying LFO to amp at extreme '+' or '-' settings causes a grating noise which is obviously just the feedback circuit kicking in and normal behaviour.
With 'Brute factor' at zero it is completely smooth.
EDIT: By the way, the forum 'New Topic' button appears to be back...