Engine Sound At 6000-7000rpm

[Oblivion]

I have an r32 gtst, with a stock engine and everything else, apart from the front mount.

at about 6000-7000 rpm it sounds wierd, not pinging, but kinda rough sounding.

the car has stock exhaust, the only thing upgraded is the plugs really, a colded one.

but it has not always sounded like it does now.

i was thinkin that it could be the O2 sensor as that hasnt been changed and it is running rich.

Any thoughts

[R31Nismoid]

O2 sensor doesnt affect things, thats only for cruise.

It could be a leaky gasket?

A vibration noise?

What type of noise is it?