i wouldn't drill any holes in it. what i did was:
at Ace Hardware they sell a pack of gasket material. it's 3 squares, one red rubber, one cork, and one grey stuff (you can use the grey to make intake and base gaskets.)
i used the red rubber one, traced out the clutch cover, and cut it to fit inside. i used clear spray paint as the glue. just spray a bunch on the inside of the cover, let it sit a few minutes to get tacky, then stick the gasket inside. i left mine overnight with a can sitting on it to weigh it down.
still working after a month. the spray paint works great.
if you want to be sure that's where the noise is coming from, just stick your hand over it while the motor's running, and see if it quiets it down.