If the leak is only noticeable at a certain psi then it might be a tear in one of the I/C pipes or a bad coupler. Cap the compressor housing and pressurize the system with a air source. Ideally, I would use one of the vacuum hoses that can be found on the intake horn side to pressurize the system. If you don't hold any pressure then you have your answer right there... Now use soap water to pin point the leak as some one else stated.