Speakers pop when volume knob is turned

I have a MF A300 connected to a pair of Thiel 1.5s. The amp is also connected to a REL subwoofer via its high level Neutrik Speakon cable. Problem is, and this has just developed, my speakers "pop" or "crackle" when the volume knob is turned--no music playing, just the turning the knob. Is this normal? I never heard it before. Should I just play ignorant and just not turn the knob without music. Or is the pop indictitive of some impending failure in the amp?
You probably have a dirty volume potentiometer. A proper cleaning would fix it, but I would not attempt that yourself unless you know what you are doing. Some cleaners can dissolve plastic! A temprorary solution would be to spin the pot back and forth a few times with the power off of course. The crackling could damage a tweeter if it is severe enough so either get it fixed or be very careful when changing volume.
Thanks Pmotz. The situation has worsened, unfortunately--it no longer pops, it tears. Must be one dirty potentiometer. I shut down the system and will take the amp in for service.