hfam
Inspired
@Greg Ferguson shared a tip a while back that has been very helpful, perhaps it would help you to confirm your suspicion?Definitely not cone buzz as it shifts positions in the stereo field from the middle, dry guitar tone then out to the left and right. Sounds more like clipping somewhere in the signal chain
Using the front panel:
Layout -> Zoom -> tab over to Meters
I love this feature (thanks again Greg!) You can see levels through the preset. In your case it may show if there's a an unusual spike in level somewhere causing subsequent cliipping.
Just a thought, hope it helps.