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IME crackling usually means you're getting XRUNS
that is that the next sample is arriving while the machine's still processing the previous one, and it gets dropped. This results in two hard pops at each end of the sample as the wave slams to zero for the length of the sample and then slams back up to whichever value the next sample has. This usually doesn't only happen once so you end up hearing a series of hard clicks/crackles.
If you are experiencing that you can try a few things. First would be increasing the size of the input buffer. First step is to bloat it all the way out; if that doesn't fix it you're very likely to be dealing with a fault in the unit. Yes that'll mean serious input to output lag, but afterwards listening to the recorded track will tell you if that's an issue; if the crackles disappear that means you'll be wanting to look at the computer instead of the unit. If that doesn't help, maybe open it up and see if there's dust in there causing the chips to overheat?
Crackling and xruns are a large part of the reason why I'm highly highly sceptical about getting it to work with an adaptor....