you can adjust the valves it it may minimize it, but the M20 does not have quiet valve operations. they are kinda supposed to sound like a typewriter.
it sounded fine to me, and i have had two M20's. if you can, try taking an audio only clip with a higher bit rate, and while your under the hood listening, open the throttle a little bit, just pull it open with your hand. not too much now. . .