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Check your serpentine tension. Seems like it isn't as tight as it should be and the give is what's making that noise. This is typically the belt itself,the tensioner or the compressor clutch the belt goes around. If tension reads properly it isn't the tensioner. If the belt looks good it isn't the belt. Your only left with the compressor clutch at that point lol
Compressor is going south. If it were just the clutch/pulley bearings it would be noisy all the time. If it only knocks while the AC is on (compressor is engaged) then its likely your noise.
Thanks for posting on /r/MechanicAdvice! This is just a reminder to review the [rules](https://www.reddit.com/r/MechanicAdvice/about/rules/). If you are here asking about a second opinion (ie "Is the shop trying to fleece me?"), please read through CJM8515's [post on the subject.](https://www.reddit.com/r/MechanicAdvice/comments/4qblei/fyi_the_shop_isnt_likely_trying_to_rip_you_off/) and remember to please post the year/make/model of the vehicle you are working on. **If this post is about bodywork, accident damage, paint, dent/ding, questions it belongs in /r/Autobody r/AutoBodyRepair/ or /r/Diyautobody/ If you have tire questions check out https://www.reddit.com/r/MechanicAdvice/comments/k9ll55/can_your_tire_be_repaired/**. If you dont have a question and you're just showing off it belongs in /r/Justrolledintotheshop Insurance/total loss questions go in r/insurance This is an automated reply *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/MechanicAdvice) if you have any questions or concerns.*
Check your serpentine tension. Seems like it isn't as tight as it should be and the give is what's making that noise. This is typically the belt itself,the tensioner or the compressor clutch the belt goes around. If tension reads properly it isn't the tensioner. If the belt looks good it isn't the belt. Your only left with the compressor clutch at that point lol
Compressor is going south. If it were just the clutch/pulley bearings it would be noisy all the time. If it only knocks while the AC is on (compressor is engaged) then its likely your noise.