Coghlan's Bear Bell with Silencer
The Coghlan's Bear Bell attaches to clothing or pack with a velcro strap. The movement you make while riding will cause a steady ringing to warn animals or nearby hikers of your presence. The magnet inside the bag eliminates any noise.
I'm on my second one after a couple months, due to the first one ejecting from my bars at some point on a ride. In my experience it's impossible to get it to lock down on your bars with the velcro straps, and they do seem to loosen up. Mind you I pretty much ride on rocky, bumpy trails, so someone who rides more XC-type terrain may not experience the same issue(s) I did. As far as its noise, I don't remember ever coming up behind a hiker or other mountain biker where they didn't hear me coming. After I lost the original one I bought the other one they sell on here that doesn't have a bag, but instead a magnet sewn into a velcro strap. My experience with that was when you removed the magnet from the bell and attached it via the velcro to the tightening strap, the magnet always ended up migrating back down to the bell and silencing it anyway, so IMO the ONLY real option is this one. FWIW, it may seem like it'd be a PITA to get the bell back into the bag as you're riding, but it isn't for some reason. It DOES still make some noise however, but it seemed to bother the people I ride with more than me. I easily tune that kind of crap out lickity split. Bottom line is that I would-and am- buying it again.
Most of my rides are on the weekend a.m. which is usually peak time on the trail. Me and all my friends ride with these a good way to alert hikers.