KXD15 loud buzzing sound

Contributor - Level 1
I own a Behringer KXD15-UL amplifier. Very often, but not always, after a few minutes of use, it makes a horrible, *really loud* buzzing, crackling sound. The buzzing sound only goes away when I turn down the Main FX level or the Main Output level.

Has anybody experienced this? Any advice? I've gotten an RMA number from support to ship the unit back for service. But, I wonder if anyone has seen this before. This is my first Behringer piece of gear and I'm very disappointed.