Fender Bassman 200 makes whooshing sounds

I have a Fender Bassman 200 that makes a whooshing sound similar to a breeze or wind blowing across a open hot microphone with or without anything plugged into the input jack.

I got this amp from a friend of mine because I was a bench tech for years, but now retired.

I just downloaded the schematic for this amp but the print is too small to read. 
Any help would be greatly appreciated.

Thanks in advance.