Buzz/Whooshing Noise (From Preamp?)

I am experiencing some noise coming through my speakers that I think is coming from the preamplifier, which is an Audio Research SP-9 (a tube-SS hybrid) and is about 30 years old.  On the left channel, it is a high pitched buzz coming from the tweeter; on the right channel, there is a whooshing noise almost like listening to the run-in groove on a record.

At first I thought it was some slight buzz/hum from my turntable (Rega) as it has always had some issues.  I recently noticed that I could hear the sound even when the turntable was off and the selector was on "spare" which I use for streaming from my DAC. So I disconnected the turntable and unplugged it and can still hear the noise.

This is what I have diagnosed so far:

1.  Muting the preamp removes the noise.

2.  Switching the reverse, right, or left switches has no effect on either sound.  It does not diminish or move from side to side.  Similarly, the selector switch has no effect - the noise remains regardless of the input selected.

3.  Swapping tubes has no effect on the noise.

This makes me think it is something in the caps or other electric part of the preamplifier.  Any thoughts?