I'd like to know more about this coil under the dashboard... Occasionally I hear a wheezing sound there, goes away when the HVAC is turned off. Air in the system? Needs a recharge? Everything seems to work normally otherwise... Any ideas?

 

He's referring to the AC evaporator coil - it's part of the AC system and is basically a radiator-type device with very cold refrigerant that the air blows through. This cools the air before it comes out of the vents.

This is likely not related to the noise you're hearing - the coil doesn't have any moving parts, it just has refrigerant pass through it. Often, the blower motors begin to squeak or collect debris, which can cause noise. This is just the fan that blows the air through the vents, so it makes sense that the noise would stop after you turn off the HVAC. You can get a new motor for $40 on RockAuto, and others have had success with taking theirs apart, cleaning and lubricating the motor, and reassembling it. This piece is relatively easily accessible from under the passenger side dashboard (if I recall, you don't need to remove the glove box - you can just access it by removing the kick panel on that side).

 

Thank, however I have already cleaned and lubed the blower fan. This noise sounds like air slowly moving through pipes and comes from behind the radio.

 

The pressure in the AC system is too high for air to leak in - the refrigerant will leak out, and it will no longer be cold. If it's a gurgling/liquid rushing noise, you could have air in your cooling system passing through the heater core.

 

I posted a video of it a while back and got no replies... Here it is:

 

I'm logged into YouTube and it says "Video Unavailable. This video is private."

 

Thanks, I just made it public.

 

Interesting. Definitely not the heater core.

Depending on the ambient temperature, the equalized R134a pressure in the AC system is about 50psi with the compressor off. There is a low pressure switch that won't allow the compressor to run if the refrigerant has leaked out. Air can't really leak into the system - the only way that could happen is if, at any point, the AC system was improperly serviced and charged up without being correctly evaporated. If the noise is a refrigerant leak, then your AC probably wouldn't be working by now.

If I had to guess, I'd think that perhaps there is something wrong with the expansion valve. Is the noise constant or intermittent? Is the AC functioning correctly? What are your vent temps vs ambient temps?

 

Thanks again! The AC has never been services AFAIK... It seems to be functioning correctly. I seem to remember it being a little weak on a really hot 100 degree day. The noise is intermittent and usually only happens for a few minutes after startup but has happened randomly while just driving around. I usually leave the AC on and not in the econ mode year round.

 

If the AC is working normally and the hissing is only present for a few minutes when you first turn it on, I'd just leave it. Not sure how much you know about how AC systems work, but I can give a high-level summary to explain the recommendation:

The system is filled with refrigerant gas, and there is a high-pressure side and a low-pressure side. The compressor compresses the gas into the high side, where it runs into the condenser in the front of the truck (in front of the radiator). The refrigerant gas gets hot as it is compressed, and in the condenser, the hot refrigerant is cooled down. Then, the cooled, high-pressure refrigerant is fed through a pipe to the thermostatic expansion valve, which steps down the pressure to the low side of the system. When the pressure is reduced, the refrigerant becomes very cold, and is fed through the evaporator, which is where the air is forced through. This cools down the air that comes out the vents. From there, the refrigerant travels back to the compressor where it continues going through the system.

The valve can stick or fail, which can cause permanent hissing and prevent the system from maintaining a sufficient pressure differential between the high and low sides for adequate cooling. However, the valve is a huge pain to access - if I recall correctly, you need to disassemble the whole dash and drain the cooling system. If the hissing is only intermittent and the AC system is still keeping the cabin cool, then I wouldn't bother.

If you want to diagnose further, you could purchase/rent a set of AC manifold gauges, which will allow you to read the pressures on the high and low side of the system to see what the differentials look like. This said, you'll need to do some research to figure out what they should be, since it depends a lot on the ambient air temperature and quantity of refrigerant in the system.

Edit: if your engine was replaced, then at some point the AC system must have been evacuated and then refilled.