When was it last serviced? If its been a while, I'd go for this as a first port of call. Change spark plugs and leads too. Obviously check all vac hoses and breather pipes, and make sure they are all firm and intact. Get someone to rev whilst you look over, and try and pinpoint the noise. If you can see the breather pipe compressing whilst at revs, replace it. Dizzy could also be at fault for the misfire, and a few others.......