System start-up noise

Happy Super Bowl Sunday,

I have a question about what may be causing some noise my system is producing just after start-up. Please note that all the components involved are always in stand-by. The components involved are a Sonic Frontiers Line 1 preamp, and Parasound JC1 mono amps. The speakers are Triangle Volante 260.I always bring the preamp out of stand-by first, it has a thirty second mute process while powering up. Then I bring the Parasounds out of stand-by, while the Line 1 is in the mute phase.  If I don’t put any music on, I’ll hear a muffled distortion-like noise from one or both channels after about 3-5 minutes of the system sitting idle. The  noise isn’t loud, and has an intermittent quality.  The first time I noticed it I wasn’t paying attention to the stereo, and at first thought a squirrel had gotten into the eaves and was scurrying about.  The noise had that quality of muffled scratchiness. After a few minutes, the noise goes away, and doesn’t return until the next time I bring the system out of stand-by.  I’ve never heard the noise while playing music. The Line 1 has has three pairs of 6922 tubes, could these be the cause?  I bought the unit used, and I have no idea how old the tubes are. 

Thanks in advance for any advice, 
Dave