Loud Strange Sound At 4000 Rpms

I have a 95 gsx and they had just rebuild the motor and when I rev it up around 4000rpms the motor, right in top of the valve cover makes a strange loude noise. I dont know if its normal or not but what do you guys think.

is it a nocking noise,grinding noise need more information

is it a nocking noise,grinding noise need more information

Its just a loud engine noise like a "VROOM" thats all I can say I dont know how to explain it any better. YOU know that if its a grinding noise its going just to be on a particular part or if its a knocking noise its not going to be the whole engine. Well my car its the whole engine that it does that. IT just changes different when your at 3500 and when your at 4000

mabey its your turbo pushing more air out of your exhaust there for makein it louder