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Discussion Starter · #1 ·
A strange knock, timed (as best as I can tell) with the AC compressor turning on and off, occurs intermittently (while parked and maybe once per week). Yesterday I was parked for a while (10 min) waiting for a call when it started. There will be a metallic/dull knock and the tach will show a 200/300 rpm blip. Then about 10-30 seconds later it will happen again (always in pairs). I decided to record it but inside the cabin it is too faint for my phone to pick up, and each time I popped the hood the noise stopped. Although, the first time I noticed the noise, 4 weeks ago, I was able to hear it more clearly on the passenger side of the engine bay. Additionally, I tried putting it into gear and when the knock occurs the car wants to lurch against the brakes. As soon as I start driving the noise goes away for days up to a week.

While I was at a dealership picking up some parts I ran this by one of the service guys, but he didn't have any suggestions other than to learn how to reproduce it. So far it's only been parked, car is turned on... and maybe it will occur, most likely it won't. I don't usually sit in my car for a long time after turning it on, so that might be the key to reproducing it.

Any thoughts or suggestions?
 

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Discussion Starter · #3 ·
Just shy of 120k. No engine lights. Yes, the plugs have been changed. However, this sound is not like the engine knock due to timing that I experienced on an older car, and there have been no performance issues. It sounds external to the block and I've only noticed it while parked at idle.
 

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Has your compressor blown yet? It may be a sign of failure since you are noting a relationship between the rattle/knock and a/c cycling. Try toggling a/c manually and see if you can reproduce the noise.

Also, drive a few days with the climate control completely off and see if it happens.

The balance shafts and chains chatter a bit at idle, more so when there is a light load on the engine like A/c, headlights or other heavy current draws.
 

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Also check the belt auto tensioner as it could also be making rattling noises. Someone on here has reported that problem where the belt tensioner rattles when the AC is on.
 

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Discussion Starter · #6 ·
Thanks for all the replies so far.

In southern Texas one doesn't usually voluntarily choose to go without AC. However, the weather just cooled and I'll give it a shot. The compressor hasn't been replaced, and I haven't noticed any AC degradation (or overcompensation, which can happen on some AC units when the pressure drops).

I'll try wasting some time parked in my car this week (burning gas) to see if I can nail down if manually triggering the AC has any effect, and maybe a more specific location.

I recently replaced my alternator (between first noticing the sound and posting here) and I didn't feel any looseness in the tensioner or other pulleys when I checked.

I will repeat the one detail that has me stumped: the engine speed will blip when making the noise which makes me think either the ECU is preparing for a load that doesn't occur, or there is some intermittent load spike. Again, I run with the AC on ALL the time, and this noise has been very rare. If it was the compressor, I would expect to hear it from the moment I turn the car on till I turn it off.
 
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