2005 GMC Envoy - there is a whistling noise in the inside, rear part of the truck. I think it is the compressor, where you can put a hose to fill tires with air pressure. Any ideas? thanks!

Noise occurs when ???

Take out the cabin air filter that is in the plenum behind the glove box. This will lessen the noise but you should get the unit serviced soon.

There are several. They go to the transmission, the idle control valve, the fuel cannister, cruise control fuel pressure regulator and inside to the heater/ ac controls There is a place on the intake manifold where a vacuum hose will connect to that distributes vacuum to all the other places vacuum is needed.

Its inside the connection where the suction hose connects to the evaporator. You must release all the freon out before you open the hose or you'll get a face full of compressor oil.

You should have 2 "round things" there, those should be the front and rear pumps. All it is is a 90 degree rubber hose fittings that goes into each one. Maybe this hose is cracked or off.

Don't think you can, even if you were to put air from and outside compressor there is no way to keep it from leaking back out. If the compressor is bad, you really have no option but to replace it.

the first thing i can think off is the secondary air injection pump,this makes a noise as you stated, and does so on start up's.

Just behind right rear tire up in the wheel well. Air inlet is inside right rear quarter near the accessory inflator. Compressor runs 3 lines one for the accessory inflator and one for each air bag.
THere are also 2 sensors, one on each trailing arm that senses load and then raises and lowers accordingly.

You have a leak in the air system. You can replace it with regular suspension parts.

There may be a small hole or cut in a vacum line or possibly just loose. Also make sure the air filter box and the big hose to it are secure and clamps etc are tight. Pulleys and belts can make a high pitch sound also.