I had this sound the other day when I fitted A RX18 board to my jagdtiger and it was the trims that were not centred so check them hope this helps.
Stevie
Hi Its the FETs on the RX18 they work at a higher pitch ,There is no cure unless you change to RX14 ,it can get even worse if you fit a DBC/dbu it feeds back through the sound card,but there is a fix for this .