not even a bang really, heard a small tinging sound. The only thing I had heard before that it reminded me of was a serpentine belt starting to separate and small pieces slapped the hood as it spun. Then heard a pressure vent sound and realized something more serious was amiss, before I could get off to check the pressure sound I started to smell the antifreeze. I was just under the hood changing filters the day before, and didn't notice anything amiss with the fan. Although it could have been cracked slightly and I wouldn't have noticed.
edit: I don't think it could have contacted anything because the only other part of the engine bay that had any noticeable wear besides the fan and radiator was the alternator pulley which just had the paint scraped off the outer edge from the chunk bouncing around. Still wish I could have found the other piece of the fan, but I heard it pop out as we were slowing to a stop and it was gone along with the sound of it tumbling.
