I found three in my last game...so not that rare. Yeah, it is a pretty cool sound effect though. I was disappointed because they didn't fit into my build so I kept LF(lutefisking)'ing them.
This...is...Caketown!!!!!!! I love that shield. I keep it in my inventory, drop it occasionally, and then pick it back up to listen to that sound clip (and annoy my son). As for being rare, I think I've found at least 2 on every character that was able to make it to Levels 7-10. So I'd say it's fairly common in the deepest levels and pretty damn rare at the higher levels.
