Ken, I am not a big expert on FSK, but that problem has cropped-up on the TS-480 from time to time.
It is caused by one having a stereo plug in the TS-480's key jack. Changing this (or using an adapter)
to a mono plug solved that problem. Note that the stereo plug works fine for CW on its own, just causes
the FSK problem when you want to use that mode.
If that doesn't help you, then I will have to pass to someone with better knowledge.
73,
Mitch DJ0QN / K7DX