My M13 Is Making A Swirling/warbling Noise in M5 / M9 / M13 Posted February 25, 2014 Well, were you using it in the loop before? It is a parallel loop, and, honestly, I've seen dozens of guys complain here over the years about not being able to use the M13 in the effects loops of Mesas. I suppose there are some effects on the M13 that might work OK in the loop. Particularly those that allow the mix to be 100% wet. Most of the mods, delays, and reverbs. Although with that, I'd think only the mods would be useful. Yeah, it's always been in the loop. I have it assigned to only be in the clean channel and I don't notice the phasing as much in the clean channel. But this is not phasing at all, it's a constant like 1000khz squeal that sounds like it has a circular motion to it. EDIT: Another thing to add, if I turn the amp on and don't play anything yet, the sound isn't there, but if I make any noise at all with the guitar, the warbling sound slowly starts to fade in until it's full blast much like feedback from a PA system would. You know how it starts faint and then quickly builds on itself exponentially until it's a full blown squeal? It's just like that except it sounds like it's going through a rotary speaker. And this is with no effects on whatsoever. So it's very reminiscent of a feedback issue. I have yet to try the M13 on another amp because I have it rack mounted on a drawer in my flight case and it's not easy to get to. But I'm almost positive it's the M13 with the issue.