Here's an odd one. I bought some new vacuum hose, a new check valve(it's on the passenger side of the plenum. a vacuum hose connects to the plenum, then the valve, then splits off to hvac controls and the cruise control. it's positioned roughly between the map sensor and the coil) and a new charcoal canister. I replaced the canister and the valve and replaced the hose running from the cruise control to the check valve and a 1-2 inch length of hose from the check valve to the hard vacuum line leading to the hvac. I didn't replace anything else, mostly due to not having the correct hose sizes. Now, the 4 inch or so piece of hose going from the check valve to the plenum is in bad shape, with numerous cracks one or two of which go through the hose. It's a bit loose on it's connector on the plenum but everything still works, air even seems to have more volume coming out of the vents.

Anyway, knowing all that. The problem is when the air is on and I turn the car off with the air on, this... squeal seems to be coming from the compressor area. Now by squeal, I don't mean the usual type. I mean it sounds like someone has a balloon pinched off and it slowly letting the air out. You know that high pitched sound. It sounds pretty much like that. It lasts a few seconds and basically sounds like letting the air out of the balloon completely (gets lesser as the few seconds pass) and this is AFTER the car is off and the belt is no longer turning anything. You don't hear it when the car is on or the air is off. Only when the air is on and you turn the car off. It seems to be in the compressor area, but there are vacuum hoses around there and also that bad hose I mentioned above a bit further away. This only started AFTER I replaced those couple hoses and the canister and check valve(ac check valve I think it's called.)

I can't see any obvious problems. All hoses are connected, I didn't remove any hoses in the replacements except the ones attached to the canister and the check valve/plenum/cruise control. Nothing seems to obviously be screwy. Air works fine, etc. Everything also worked fine before these "maintenance" replacements. Does anyone have a clue where this sound is coming from and why it only showed up after I replaced those items?

 

Sounds like a vacuum leak to me. Get a piece of garden hose about two feet long and put one end up to your ear and use the other end as a microphone to find the squeel. Have someone start and shut of the engine a couple times and you can pinpoint the noise location pretty quick. Crude, but it works.

 

I kinda figured as much, but I have no experience with vacuum leaks. I almost bet it's that worthles 3 inch piece of vacuum tube that's gone south. Regardless, I'll be replacing ALL the rest of the vacuum tubing under the hood this weekend and we'll see if that fixes it, then I'll check for leaks if it doesn't. Thanks for the pointer in the right direction.