[Mondial]

Sounds like you might have a leaky coupling cap C304, (.03 uF) which passes the audio from the FM detector to the volume control. If this cap is leaking it will put DC voltage from the plate circuit of the detector into the volume control pot, making it noisy. Since you don't get the noise on AM or SW when C304 is disconnected, my guess is that the cap is bad.

You can confirm by connecting a DVM to the test jack J200 on the back right of the chassis. This test jack connects directly to the top of the volume control, so you can monitor for DC voltage as you switch from FM to AM. There should be no DC voltage at that point on FM, SW or AM.

If there is no DC when in FM mode but the pot is still scratchy, then there might be a possibility that here is an ultrasonic oscillation or RF on the FM detector out. You could check this by connecting an oscilloscope to J200 and looking for any unusual signal there.

In any case, the volume control should not be noisy in FM position, as mine is equally quiet on all bands.