It could be over heating (at least that's what I think) I recently went on and put my mojo on an 8 hour marathon playing and simultaneously charging it and it sort of emitted a sizzling noise. My theory is that the glue inside holding the components together is boiling or something but I could be entirely wrong but that's the most plausible cause I've concluded. Try running it in an air conditioned room (I'm in Singapore so the weather is made for frying eggs) But if you're in a country where you piss icicles then you can kindly disregard this
A couple of times when I used Mojo playing and simultaneously charging from low battery, I overheated the device and the security circuit had shut the device off. (I don't have this problem anymore since I keep Mojo nonstop plugged in. [And this can happen only if you play and charge from low battery.])
I never heard any sizzling noise from Mojo. And it did get hot often for prolonged listening times. (It does have a hot electric smell though, if you smell its holes closely, but nothing abnormal. I kinda like that light smell. But no noise at all.)