Recently bought 2001 3.8L Park Ave with 84,000 miles. Original owner had to enter total care center. Car looks and runs/drives excellently.
Two concerns to query your minds...
1. When I turn off engine, whether running just a few moments or an hour, I hear a brief "pssss" sound from near front of engine. Similar to an air tank leaking down for maybe 10 seconds. Cannot spot source but never heard this on any other car in past. Any ideas that may be particular to this model?
2. I noted that the coolant bottle holds fluid at constant level, but having heard a water gurgle sound from under dash a few times I suspected air in system and removed the radiator cap -- about a quart low. I removed reservoir and feed hose and found the hose blocked by a grainy reddish brown residue for about eight inches near the reservoir. The drain nipple on the reservoir was also plugged, so no coolant was passing from reservoir to radiator.
The residue is like finely ground damp pencil sharpener cuttings. Soft, but does not act like a pasty gook.
I am suspecting I may have the dreaded intake manifold leak. Engine had never run hot and has no observed external leaks. I have read the December 2014 thread started by Kimberly - great thread by the way - and am thinking this is in the very near future. Any other ideas of probable cause?
If I indeed must do this gasket maintenance now, I guess I will also be replacing the serpentine belt and tensioner and hoses while I have it apart.
I have cleaned out the hose and reservoir this evening, so do not know yet whether this blockage was causing coolant to leak about the radiator cap as an escape route (since proper overflow not possible). I'll know in a couple of days whether all air is bled.

Will certainly appreciate all words of wisdom and experience.

Someone before you probably mixed some other brand of coolant with DexCool. Not cool.

If you don't know the complete history of the car, then I highly suggest in order of importance...
1. Lower/Upper intake gaskets. Don't get the cheap ones, get the metal framed lower gaskets.
2. Upgraded aluminum coolant elbows.
3. Oil change will be required after the intake system has been done.
4. Flush the cooling system with chemicals, not just water. Then just top off with a normal coolant.
5. Full tune up. Wires and plugs.

Basically what you found, was air mixing with Dexcool, due to a leak....whenever you have a leak, you create an air pocket...air and Dexcool do not mix well....it will form an orangey sludge, usually on the bottom of the radiator cap, and in this particular case, probably got forced into the line to the reservoir, during expansion of the coolant......

Recommended coolant change is 5 years/150K MILES, which ever comes first....which I don't agree with......but even by those standards, this system should have been flushed/replaced twice already, and probably has original coolant in it....

Agree with Mike and Tech II, David the OP mentioned a grainy, brownish sludge which also might indicate that someone may have tried to address a coolant leak with those GM "stop leak" cooling system pellets which I believe were nothing more than ground up walnut shells.

2004 Bonne SLE
1971 Chevelle SS

Ahhh, the famous "mechanic in a bottle".

And I wouldn't be too concerned about the noise mentioned. It'* likely either the A/C pressures equalizing, or possibly the vacuum reservoir for the HVAC leaking down.

rjolly,
You are right.... About dinner time I listened closely in a quiet area and the sound really seems to be coming from the AC system. I'll put this concern on the back burner and just "watch" for awhile to absolutely verify.

to all,
After cleaning the hose blockage and driving about 15 miles total, the system has purged air and I do not hear the gurgle from under dash. Still too early to know it that was only problem. However, to be safe, I am putting together a list of parts to order for replacing the UIM, and both upper and lower IM gaskets. Will include all hoses, plugs, wires, thermostat, tensioner and serpentine belt. If the gaskets sets do not include the aluminum elbows, I located them shelf item at Advance. Meanwhile I will drain/flush/clean the cooling system, then replace the coolant. I plan to use extended life Prestone, not any Dexcool product.

Any probability that I should order a lower manifold also, or is this typically a no-problem item?

No, the lower manifold is trouble free. Just the upper plastic plenum and associated gaskets.

Thanks, Mike, for the 'good' news on the lower manifold and also for all your previous comments. Will get my order out to RockAuto this afternoon. Sounds like the install itself should be uneventual, so I will report results when I get task complete. Again, thanks to ALL for the comments/advice.

Also I wanted to add, many find the the casting of heads and LIM to be poor, and much benefit is often gained when they are touched up (or full on ported) while the vehicle is apart. Now, this isn't for everyone, as you are admittedly shaving metal off of your heads and manifold, and quite possible could drop shavings in to the cylinders if you make a wrong move. However, it is worth it if you have a sure hand and know what you are doing.