Fog horn noise after getting my AC charged

I have a 2003 Subaru outback
I got my AC charged since it was not cooling. As soon as I left the shop I had a fog horn noise coming from the passenger side of the car. It comes and goes and even occurs when the defrost is on. Also the car runs rough when the noise occurs. The roughness stops and the fog horn noise stops when I turn off the AC or defrost. I took it to my mechanic and he has never heard that noise before. It really sounds like a fog horn.

Your mechanic overcharged the A/C system with too much refrigerant.

Take a look at this recent thread regarding the exact same problem:

I was just about to say the same thing happened to me recently and what it was. But the link above from VDCdriver is of my post. Good luck!

i know its hard to provide details of repair. did they add freon or did they evac system and add back the full amount listed on service tag? such as 1.2 lbs or 1.6 lbs or something in that amount?

Thanks, I will take it back and have them check it.

Just to let you know they had overcharged the A/C. After they fixed it the car runs perfect with A/C working. Thanks. I was not charged for them to fix their mistake.
Thanks for the advice.

Good deal. Thanks for the update. This might happen again to someone else later on and it is good to know what solved the issue.

My 2002 Outback has been doing the same thing when I turn the A/C on. It makes a continual horn / quite foghorn / elephant - like sound shortly after the A/C is on. The horn-like sound continues even after the car is turned off. A/C still seems to work. I’ve had the car for about 4 years and have not had the A/C recharged so I’m wondering why this is all of the sudden happening? Thoughs?