Maybe it's not farting, but sounds like it? Have you tried checking all the screws to the speakers and the grill? Or checking to see if the handles are rattling? Or maybe even the GK or NEO logo could be vibrating? Although I don't own a NEO 212, I have played them before and have not experienced this, that's why I am offering some recommendations.I hate my GK212 Neo, bought new, never gigged always low volume.
farts all the time if bass is barely boosted, low gain, low volume....new batteries different cords.
can not do a low a...
Finally went in the garage last night and dug out my ancient GK210 backline combo. No farts with tons of bass boost, high gain, high volume not a problem. Low A was great.
Mines going on crags list this weekend
I do own the NEO 410 and NEO 112 and I don't experience any farting on either of those cabs as well. Hopefully it is a simple solution that you can find before getting rid of it.