Hello, i've noticed that the the when the power amp is on (doens't matter which one) that it seems to make a crackling noise.  It is hard to hear when playing, but when you let a chord or note ring out it is noticeable.  I have determined that it is the power amp function as when I disable it the noise goes away.  Is this normal as it is modeling power valves or is there something faulty?  

Thank you.