Pre amp noise

How good is the power in your area? are you using some sort of power conditioner?

You may need to clean your selector switches and/or volume controls. Next time it happens, power down and move the controls back and forth. You should also clean the tube pins and sockets.
Are you sure it's not crosstalk? Whenever I play vinyl, I can faintly hear the tuner if it's on. Same thing happens with CDP.

Tough to give advice..need to know what type of preamp and source ? vinyl,digital? does it only occur in the right channel? If you switch tubes does it move to the left? Tubes will get noisy when they want to be changed..maybe thats it? or maybe you just need to run a ground wire from source to amp.