Hissing sound when Tone Control is turned on

Hello all,

Wonder if any one of you experienced the same issue with your Mcintosh preamp ? If you did and what was the solution to it?
I have noticed that apparently audible hissing sound can be heard from my speakers during idle while the "tone control" is enabled on my Mcintosh C49 preamp. As soon as i toggled the tone contol switch off from the front panel the hissing sound is completely gone and the background is dead silent like it supposed to. 

Is this considered normal ? Any advises will be appreciated.

Thanks

Larry