I would not return to that shop. I hope you didn't have to pay for that "advice". For starters cassettes don't need to be 150 bucks to work right when set up properly....
Without knowing anything else, my best guess is that the shift cable is out of adjustment, holding the derailleur "between gears" and the chain is hopping back and forth between them, making the popping noise as it does. The length of the shift cable determines where the derailleur is positioned, and there's an adjuster at each end (near the shifter and back on the derailleur) that can be used to make minor adjustments to the cable length. Usually a little adjustment at either end goes a long way. 1/4 turn can make a difference!
I would highly recommend grabbing a cup of coffee and taking the time to brief yourself regarding the derailleur adjustments. YouTube has dozens of them. I see this one mentioned frequently:
The article will discuss adjustment of rear derailleur limit screws and indexing.
www.parktool.com
Looking for something quick and dirty? Try turning one of the adjusters (either end) 1 full turn (no more) to see if the problem gets better or worse. Some adjusters will need to be pulled out slightly before they can be turned. Don't just put a pair of pliers on it and force it. This should be able to be done easily with just your fingers. You should see a significant difference with that full turn.
If that does take care of most of it, try another 1/2 turn one way, then the other to see if it improves. As mentioned, this is the quick and dirty method. For Pete's sake, don't take it as gospel. You should still familiarize yourself with the correct procedure -OR- find a competent shop that knows what they are doing. Techs talking about "cheap parts" are exposing their complete lack of knowledge/competency. Ignore them! Plastic pulley gear........right...... went to two shops and again the bike shifts fine and I have no really problems shifting. the clank comes well after