A month ago, I had my AC serviced. It had no pressure and they had to add R134a plus a sealant. No other repair was done.

 

A week ago, the AC started making a harmonic, vibrating sound. The sound occurs every minute or so once the AC has been on for a while. It lasts about 2-3 seconds. It sounds a bit like a horn or a harmonica, starting loud and then slowing fading out. The sound continues for 2-3 seconds after the car engine is turned off. It also sounds a bit like the sound you'd hear from the harmonics when a house hot water line is vibrating.

The AC low pressure line reads 25psi. The car is a 2000 Subaru Legacy 4 cylinder. When it making the sound, the AC line vibrates from the compressor to the evaporator to the receiver/dryer. It's a very noticeable vibration. I can feel it with my hands.  

 

It's not possible to pinpoint the exact origin of the sound, but it's less noticeable at the compressor. It's more noticeable at the top of the engine near the air filter housing. I've removed the AC housing to see if it was vibrating, but the AC noise continued. I think the line itself is vibrating, but when I grab the line with my hand, the sound continues.

 

Other than this noise, the AC seems to work fine. 

 

My car is a 2000 Subaru Legacy Outback wagon with 5 speed and 4 cylinder engine. 

25 is a little low. The system is probably still leaking. The low charge can cause the expansion valve to make noise. Also possible that the sealer they put in may have clogged the expansion valve. The expansion valve is at the end of the high pressure line right where it goes into the AC box at the firewall.

 

Do you feel the vibration on the low pressure line (the large one) or the high pressure line(the small one)?

The low pressure line (big one) vibrates going to the evaporator at the firewall, coming out of the firewall to the dryer (I think it is called a dryer), and coming out of the dryer going to the condenser in front of the radiator. The line (little one) from the condenser to the compressor doesn't vibrate at all. 

 

How low is 25PSI? I have a can of R134a with sealant and a pressure gauge. I can top it off. How much should I add?

Edited May 8, 201411 yr by jgradyc

Depending on how warm it is you should have between 30-35 psi on the low side. This alone doesn't tell you much though. It seems you have an issue on the high pressure side. Either at the expansion valve or the drier.

 

The expansion valve could be clogged or is frozen due to moisture in the system. You say there was no other service done besides adding a sealer and filling with refrigerant, so I'm assuming the system was not evacuated and vacuumed to remove moisture. Depending on how long the system was empty before, the drier could be saturated and is not able to retain the moisture that may have got in.

 

You'll have to find the source of the leak (check the bottom corners of the condenser). You can buy leak detection kits at the parts store that come with UV dye and a UV light, but usually just looking for traces of oil (appears green) is enough to pinpoint a leak on these. Then I would recommend replacing the drier along with any o-rings that may be leaking, and the o-rings on the compressor. Then have the system vacuumed before refilling.