I have the same issue and it occurs once you play more than a few notes together. Worse if you have two patches or more layered together.Chad never did get back to us so I guess he successfully eliminated the problem.
How does it sound on continuous sounds like strings and winds and synth and organs?
Is the problem isolated to only piano and clav sounds or does it occur regardless of what tone you are using?
Does it occur only on certain notes or octaves or does it range across the board?
Do you have batteries installed or are you running it solely on line power?
Is the keyboard still under warranty?
Any chance you could post a clip of what it sounds like?
Gary
I've not noticed if it happens across all octaves but I think so.
It's not noticeable if you use tones like strings or pad only because you will most likely be playing soft and not jamming