I've owned two rifles that did that. Neither had any mechanical problems. Think "tuning fork".
It's not my ears.lol it rings like a bell after the shot with sabot slugs.like a tuning fork.never had another slug gun.lol do this.
As you get older your ears will ring more and more often....
My Stihl saw does this at an idle. I've had a few others do it as well. Only heard it on a Stihl though.



























