MR816x - Electrical Buzz Issue

Sometimes my MR randomly starts generating a really loud electrical buzzing sound when instruments are plugged in. It doesn’t do it all the time, but some times it will randomly start doing it.

Is this a sign my MR about to pack in? Or is it some grounding issue with the unit? I’ve tested all my other electrical hardware and they’re fine, I’ve never had this problem before at all and I’ve owned it since it came out. I use high quality cables with shielding so there’s no issue there. It does it with all my instruments and mics… Sometimes it’ll work absolutely fine though, it’s just random when it happens.

Please help.