I'd just put a few drops of motor oil in there prefferably with an oil can so you can shoot it at the bearings, or better yet, get a can of white lithium grease and shoot the bearings directly. While you have the clutch cover off, look at the inside of the cover to see if anything was rubbing and grease the gears with a LITTLE grease- if you over do it, it will end up in the clutch. Take your time and spread it on each gear tooth with a bamboo skewer, toothpick (kinda small) or a chopstick. Sometimes (if it is the gears) the sound never goes away.
Also, looky here- http://motorbicycling.com/f30/bicycl...-care-354.html