Knocking sound when A/C turned on

Hi and thanks for using 2CarPros.com.

If it is only happening when the AC is turned on, chances are the compressor is bad or going bad. Using an automotive stethoscope, listen the AC compressor to determine if that is where the sound is coming from. Be careful because the engine will be running, the belt turning, and the compressor engaging.

If you do not feel that is the problem, check through this link and see if there is anything that helps. If there is not let me know and we'll continue checking.

https://www.2carpros.com/articles/engine-noises

Take care,
Joe

Thanks Joe.

I hope what I suggested helps. Let me know if you need anything or just have other questions.

Take care,
Joe

Good afternoon.

It sounds like you have a failed AC compressor. There may be a failure internally causing the knocking noise. Make sure the belt is good and tight as well. A faulty tension-er could also cause the issue.

Roy

https://www.2carpros.com/articles/replace-air-conditioner-compressor

What accounts for the noise when the A/C compressor is not switched on such as the time when the car was started and driven from an incline (A/C off)? Also, why does the noise not occur when on highway with the A/C switched on? Thanks.

I would start by having the belt, tension-er and front bearing checked. That may be the entire issue.

Roy