1997 Sierra...Water noise under dash

[Sumdumgi]

I started my truck yesterday and it was at idle w/o a/c on. I could hear what sounded like water rushing around in a container. What is this?

[quick69gto]

You have air in the cooling system. Top off the radiator and overflow tank. Then look for any leaks including the heater core.
The AC system will also sound like that when the refrigerant is low. If the AC compressor is cycling allot (turning of and on) with the AC set to max cooling, then the refrigerant is low.
Good luck.

[Sumdumgi]

You hit the nail on the head. The A/C wasnt working so I jumpered the switch and got the compressor to come on. I added about 2.5 #s to which the cheap gauge that came with the freon showed full. It is cycling on and off quickly though and it hasnt been hot outside.

Will filling the radiator do anything for the A/C or just prove no leaks thru heater core etc...?
If I have air in the system how can I purge that?

[quick69gto]

If the AC is still cycling at this point it still could be low or it's now overfilled. You really need to have someone put gauges on it to see what the high and low pressures read.
Depending on ambient temp (outside temp), the low side should be around 20-30 and the high side around 120-140. High readings indicate to much refrigerant and low reading indicate the AC system needs more.
High or low the compressor will cycle. The pressures have to be just right for it to stay on continuously.
The fact that you put in 2.5 pounds in a system, that I presume has no leaks, leads me to believe that it's over filled. I could be wrong though.
If the gauges show the system is still low then I would bet there is a leak somewhere.

The heater core/ radiator is a separate system from the AC system. The heater core uses engine coolant to provide heat to the passenger compartment and the radiator is designed as a heat exchanger for the engine coolant. They are both part of the same system.
Basically the engine heats up the coolant>the coolant gets pumped (water pump) into the passenger compartment (heater core)>then it continues out to the radiator to be cooled>then it continues on to the engine again starting the process all over again.
If the engine cooling system has any leaks, eventually there won't be enough coolant to keep the engine cool.
Purge the engine cooling system only after you have found a leak and fixed it, or if you had to add more than half a gallon of coolant to the system (<sign of a coolant leak).
To purge the system, fill the radiator, leave the cap off, start the truck, and let it idle. Watch the temp gauge until it reaches normal operating temp. Be aware that some coolant may puke from the where the cap usually is. Don't worry about that, just keep your dogs/cats away from the spilled coolant (it's sweet so they will try to drink it).
Once the temp gauge show it's at normal temp, feel the upper radiator hose to see if it's too hot to touch (thermostat opened). If it is, shut down, top off the radiator, and install the cap. Fill the overflow tank to the "Hot" mark and your done.
If the upper hose is still cool to the touch, then continue letting the truck idle but watch the temp gauge. It may rise higher than normal before the thermostat finally opens. Just don't let it go to the hot mark (shut it down before then).
If you do have to shut it down because it started getting too close to the hot mark, let the truck cool down for a while and try again.

Anyway, sorry for the long post.
I hope this helps.

[quick69gto]

I almost forgot, don't jumper the compressor. You could damage it.
Once you start adding refrigerant, the AC compressor will start on it's own.

[Sumdumgi]

Thanks for your help. Very good info. I will report back my findings. I need a complete set of gauges to check the upper. I will pick some up soon. I just added 1/2 gal coolant to radiator and truck is idling now waiting on it to heat up.

[Sumdumgi]

Well I never bought the gauges but my AC is still working good. I was low on coolant but so far it is holding at full. I dont hear the swishing or water noises anymore. Thanks so much for all your help!!!