Skylines and engine bay noises, do search...
When were your timing belt idler and tensioner pulley's last replaced? I've had this issue, I reckon i was a few weeks away from a seized bearing, overheated and snapped timing belt
Sure its not your injectors? If you race the motor, does the noise disappear on the decel? What oil and when was it changed last?
Regardless, poke about the engine bay with a long screwdriver on your ear.