What's this sound, and why the distortion?

[exm]

Hi!

Relatively new to REW, but I am having problems. I'm getting 40% distortion at 50Hz in this sweep of the fronts, and when I run the sweep there's a strange sound. I'll attach a short video. Is this my processor, the mic or REW?

 

[skid00]

It sounds like the woofers are bottoming out. Limit volume to 75 dB and rerun. If you still hear the clacking, start the sweep at 50 Hz, not 5 Hz.