noise...from preamp or amp???

My system:
Meridian 508.24 CDP
Melos SHA-1
Grado RS-1
Magnum Dynalab MD102
ARC Ref 2 Mk II
Classe CAM 350s
PS Audio P600

single ended ARC to CAMs, balanced everywhere else.

There is noise (hard to define, sounds like static or swoooshing) coming from both speakers. Has been driving me nuts for months. This is eliminated using the mute switch. Increasing the volume does NOT increase the sound of the "swoooshing". The sound is not there listening thru the Meridian/Melos/Grado. So I assume it's the ARC or Classe. I finally plugged the Classes into the P600. Walla! Noise greatly reduced. One night later, turn on stereo, no change from setup the previous night, and the sound has returned, seemingly at the previous volume. Now, is this a tube issue? Power cord issue? Need help. Live in NYC so power is quite dirty. Any ideas??

Sounds like tube rush to me.
You might try toggling phase invert switch from your ARC to see what happen. I have been struggling the same problem from my system for a while. But, just recently I found somehow the phase invert could help reducing the noise quite a bit. Actually, I am still trying to figure out how that works. Please refer to my post with title "Hiss and hum on ARC REFII MKI" FYR.

Good luck.
Tube rush (noise) in your ARC pre-amp. You could probably use some new low noise tubes.
I'll look into the tube rush possibility....I was also thinking could the gain be too high? If so, is it possible to lower it?
You are correct - it could be a gain issue assuming that your amp's input sensivitity is high (like .5v to 1v) and/or your speakers are very efficient. You can reduce the gain by installing in-line attenuators (I can't recommend any off hand, but I'm sure others can).