I did a little closer inspection. Here'* what I noticed...
Starting with a cold engine, using a stethoscope, I here only a little noise throughout the supercharger. No "rocks". As it begins to warm-up the noise grows and it does sound a little like rocks being knocked around but it'* not clear where it'* coming from. It seems a little more pronounced in the nose. Raising the engine speed, the rattling smoothed-out.

So, does this seem like a coupler or a bearing? Also, is a bad bearing associated with the tensioner bouncing around?

Sounds like a bad coupler. Take the belt off and turn the pulley back and forth. If there'* any play, the coupler is bad. If there'* play the tensioner will bounce a bit because the rotors are heavy. Changing the coupler is a some what easy job and laid out in a write up. If your in need of a rebuilt supercharger, just let me know, I have 4 of them.