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.