In addition to a possibly dud pot, it can be caused by DC applied across the pot, possibly from a leaky coupling capacitor. Measure the DC voltage across each channel of the pot - you may find the noisy channel has some DC while the other channel has much less or none.

DC in the presence of slight variations in the track resistance will generate intermittent AC signals as you adjust the volume, which sound like crackles or thumps.

Can a malfunctioning relay in an amp cause or allow a speaker to crackle when turning the vol potentiometer?
I'm going to have some mild steel tubes with holes drilled on it hot-dipped galvanized. I need to know how much tolerance or how big the holes should be when I drill them so that they would be at least an inch in diameter after the galvanization process. Of course, I don't want the holes too big too afterwards. 

I tried Googling the answer to this question but failed to see usable numbers. This is most probably because it depends on the size of the holes and the type of metal being galvanized. So if anyone can give some numbers based on their experience, that would help a lot.
