Gearbox Chatter

[xxxstar]

A friend of mine is haveing trouble with his Griff 500 gearbox (T5). At idle there is a chatter form his gearbox, and he wanted to know if anybody else had these problems. What oil should be used (recomended or something else). He is a mechanic, and he doesn''t want to pull the gearbox apatr to check the bearings if there is an easy fix. Thanks for your help.