Intake problems

I have a 99 gt and i recently replace the intake plenum. Now I have a high pitched whistling sound under the hood. Does anyone know what might be causing the noise. Thanks.

Sounds like you have a vacuum leak on the car. Check all the hoses and the gaskets themselves. This should correct the problem.

Another thing to check is the two o-rings in the PCV housing under the MAP sensor. When they are missing or not seated properly, they will cause a whistling noise.