That noise has happened to me a few times in the past....Just takes a squirt of cleaner in the pot. Worked for me. I don't know what the cleaner is called, the guitar tech at my local store did it......It's some kind of lubricant kind of like WD-40.

Did he try cleaning it before trying to sell you new pots (knobs)? Sound like a sales tactic to me. But, it may need to be changed out.......