Summary

Not a very loud one, but a pretty fun project. See it in action here: http://youtu.be/V6SdR8xWaw4

Instructions

I printed all the parts in ABS on a Printrbot. The gears were printed at 100% fill, though I'm not sure that was really necessary, the rest was at 40%

Other parts needed:
(2) 1/4 in. hex bolts 2"
(1) 1/4 in. hex bolt 1.5"
(3) 1/4 in. nylock bolts
(2) SR4 bearings (1/4 ID x 5/8 OD x .196) like these: http://www.vxb.com/page/bearings/PROD/SealedBearings/KIt12068
I got all these parts from my local hardware store.

Once you have everything, assembly is fairly straight forward.

update: I added a duplicate small gear as a separate download at the request of a user.

Hand Cranked Siren