bad sound comming from engine!!HELP??

[trx400ex sport]

ok.the atv is all stock,i use 93 octane gas,the noise happens in all gears except neutral, banging sound gets worse with increased RPM,s,it dosent backfire,it sounds like something smacking on alunumin,anybody have any idea what the problem is???

[Wheelie]

If it's RPM related, which it sounds like it is, the first thing I would check is the valve clearances. If that checks out ok, then there's a good chance it's deeper within the motor, I.E: Piston slap, Con-rod bearing failing/failure, etc.

If it's speed related check carrier bearings, chain slider, and the condition of the chain and sprockets.