srdavo,
I´ve checked the gears the first time i ran the engine 3 months ago, and put some loctite in the screw of the little gear because it was loose, and greased the gears a little with molybdene bisulfur (here in argentina we called molykote).
Yes, when i just put my hand softly in the gear cover, the noise is almost gone! and the engine doesnt make any other strange noises, run just ok.
It looks like the gear cover acts as a "baffle" and resonates.
I am thinking of put some silicone sealer instead of the paper gasket, or making a rubber gasket.