I had a very similar problem also with a gyro se using a stock rb300 arm. In my case it was the tonearm cable picking up an am signal, I have no idea why but I was able to virtually eliminate it by reversing the phono inputs into the phono pre (plugging the left phono plug into the right input and right plug into left input) of course then I also had to reverse the leads from my phono pre to my linestage. Don't know if that's an option but it's easy to try.
How to eliminate FM RF coming thru turntable?
Mitchell GYRO SE turntable picking up FM RF after 5 PM to late at night. During the day, no FM RF being picked up by turntable. All other components, no RF (tuner, CD, tape). All components connected thru Furman power conditioner. Have run separate ground from Furman to turntable ground at preamp (Rogue 99) which reduces the FM RF considerably, but does not eliminate. FM interference reacts to volume control. FM RF disappears when turntable leads disconnected. Any suggestions?