Car is 07 roomster 1.6 petrol auto, the car is great to drive but has one problem that appears to be a fault of some sort that i am struggling to locate, problem is engine noise, when accelerating and the revs reach 2.5-3k the engine is very noisy, its like a cross between no air filter and knocking big ends, the engine is quiet when started from cold and when at lower revs and when cruising. Its not the normal engine noise you get when the engine is revved hard and is not noticeable when the engine is revved at standstill.
I cannot imagine it is normal but if anyone has any ideas i would be grateful for some info.

Thanks Alan