Clipping!
It sounds like your level is too high going to the main mix(left/right bus), I don't know what program you used to mix this, but if you put a limiter on the mix you'll get rid of the clipping. Otherwise, turn down the levels going to the mix(don't use prefader metering if you're in Pro Tools).
Anyway, aside from the clipping, the tune actually sounds really cool and heavy. Keep up the riffing!