Noise in the impact

 Today while shooting I got a Brrrrrp instead of the regular pop of a round being fired. About 10 Rounds later I got it again. I've noticed a little up and down variation that I attributed to the regulator after all it is a FX. Has anyone else experience this noise. Anyone know what this problem is. I don't know whether you understand what I've said but if you've heard it you know what I'm talking about. I wonder if it's a flutter of the regulator, which is a good possibility I don't think it's a spring it doesn't sound mechanical although it could be.

Any experience?