Knocking/ticking noise from the A/C
 

After the car sits for a while (hour or two or more) I hear a knocking or ticking noise when the air conditioner is on. When I turn the A/C off, the sound goes away. After I drive for a while, the sound goes away. When the car was turned off I thought I heard a hissing sound from near the compressor. How many o-rings does this system have? I replaced one lat year. Maybe another is leaking and I am getting air pockets in the line? Suggestions on how to troubleshoot?

Re: Knocking/ticking noise from the A/C
 

Mine was recently leaking pretty substantially. I replaced the 4 o-rings common to the 2 main lines that connect to the compressor. There are 2 at the compressor, one on the firewall behind the snorkel, and one on the a/c condensor. You can just trace them from the compressor.

Re: Knocking/ticking noise from the A/C
 

Did you notice any knocking or ticking noises coming from the car when you first started it up and the A/C engaged?

Re: Knocking/ticking noise from the A/C
 

It would not engage, it was completely empty on r-134. Sorry.

Re: Knocking/ticking noise from the A/C
 

My mechanic thinks it may be the clutch on the compressor. It only happens after the car has been recently driven and has been sitting for an hour or so. It goes away pretty quickly once the car is started and I don't hear it if the car has been sitting overnight. He suggested letting it go for now and wait and see if it gets worse.

Re: Knocking/ticking noise from the A/C
 

if the noise is coming from the front of the compressor it could be the clutch, if it's coming from the back it could be the reed valves in the compressor. try using a long screwdriver to isolate the noise. another thing you can do to check the clutch is dump water on it when it's making the noise to see if it goes away or at least changes tone.

Re: Knocking/ticking noise from the A/C
 

The A/C is working and the compressor is not cycling, right?

Re: Knocking/ticking noise from the A/C
 

Correct. The A/C is still nice and cold and stays on/cold and especially these days with how hot it has been.