I have a RME babyface pro.  I love the interface but have a stubborn problem. When I have a guitar plugged into either input 3 or 4 everything works fine except for an intermittent hiss.  The hiss is irregular but comes at about 2-3 minute intervals.  It is constant for 6-15 seconds and then dissipates.  I have tried an external power supply, different cables (guitar and USB), inputs 3 and 4, different USB ports, and different guitars.  None of this made it any better.   If playing music from the computer, this does not occur.

I am uncertain what else to do.  Any suggestions appreciated.