I was at a birthday party and went to use my camera, except it wouldn't focus and was making a weird clunking noise when it tried

I would not rely on autofocus with this lens, as the noise is very loud, and it seems to be very slow in acquiring focus in some situations

Nothing makes any funny noises or rattles

: The lens does make a loud mechanical noise while zooming, almost identical to the 50mm 1.8 Canon lens

The lens does include a front and rear cap (the lens cap being a nice E-52II comparable cap)