There should be a crush washer around that nut. If its tight it shouldn't be that. More then likely the noise is your clutch plates rattling within that clutch basket. unless there is something obviously loose, which if there was, you would have bigger problems then a rattling sound.

A decent way to check would be to put in one of the clutch plates and see if you can rotate it back and forth within the basket. This is most likely your noise.

If you do need to tear into the motor more these are great tools if you don't have the special basket holder tool (sorry for the stupid long link) I think when I pulled mine I used this lil pro motion tool to hold it all in place and a breaker bar. That nut gets torqued down pretty good so it should be tight. DO NOT try to hold the basket with a screw driver or bar... It will snap pretty easily. (I've done this to)

http://www.motosport.com/product?psreferrer=https%253A%252F%252Fwww.google.com%252F&pssource=true&segment=badger&key=Motion-Pro-Gear-Jammer&cc=us&adpos=1o1&creative=93575775025&device=c&matchtype=&network=g&product_id=MP-TOOL-038&gclid=CjwKEAjwqJ67BRCzzJ7Hy-LYlFYSJABwp9PGbn5oh--Xm83rrErB2K0vDkAsydtb_UePD8itWTOHiBoCRyzw_wcB