[Radioroslyn]

I would suspect something in the IF or mixer stages perhaps a leaky cap or bad tube. The fm detector is designed to not detect static (change in amplitude) but a shift in frequency. So in the fm mode you shouldn't hear any static. Audio stages are probably ok as you wold hear it in both am and fm if they weren't. In terms of troubleshooting I would use a large value cap (like .1mf) connected to the b- rail and the other end  jumper to the control grid of the a fore mention tubes one by one. Listen to hear when the noise stops then concentrate on that area to find the defect.