I biked with coworkers during lunch, and everyone agreed the rattle was loud and also quite a mystery as to the source. We did our Heathrow Hills loop and the entire time my bike sounded like I had some jingle bells on it. After the ride - 16.4 miles in 1 hour - I took my bike into the shop.
I described the symptom, and also mentioned shifting was a bit sluggish on a few of the gears. Shifting issues happen all the time, through cable stretch, crud on the cassette or chain, or normal loosening. It is typically a quick and minor fix of a barrel adjustor, cable tensioning, or even limit screw adjustment (but not as often).
So the mechanic put my bike on the stand, shifted through the gears, and then poked the cassette with his index finger. The gears just mushed over... which is not normal at all! It turns out I had a broken cassette spacer, and this was the cause of both the rattle and a sticky shifting. After a new spacer, my bike shifts great and is as quiet as it should be. :)