Hehe. I'm dealing with a vibe right now that I thought was incredibly loud, but turns out just to be very buzzy & at a pitch that bugs me. So, it's nice to see someone else thinks about these things.
I don't think I'd want a separate scale personally. For one thing, "buzzy" vs. "thuddy" is a pretty subjective definition, so I think it's more helpful to have it explained in the written performance section. I'd rather have a comparison or description. For example, I am comparing the sound of this one I'm working on to a set of electric hair clippers. That brings something concrete to my mind. Also, I'm not sure the web-folk here would want to deal with adding another rating dealy when there's a good place to cover it in the written part.
I do think it's a good thing to mention, just unsure a rating bar would be the best way.