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My 2002 E320 4Matic wagon has developed a noticeable engine whine. The pitch of the whine matches the engine speed. In 2017, I replaced the tensioner, belt, idler pulley, and water pump so I don't think they're at fault. What would cause this whine?

I think this whine started after I got my A/C fixed. In August 2018, the A/C sputtered to a stop and the EC light wouldn't go off when the button was pressed. When the shop fixed it in summer 2019, they used a computer to release it from being "locked in EC mode", then added some refrigerant.

Video of the engine whine while revving in neutral.
Video of engine whine while driving.
 

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Well, I hope the a/c shop recovered the remaining refrigerant and oil in the system and refilled it per specs, INCLUDING compressor oil. Else you will have a seized compressor sooner or later.. And the whine may as well be coming from the oil starved compressor.

You can take the belt off and run the engine, and see if the whine subsides.
 

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Sounds like pulley. Remove belt and spin all pulleys by hand.
 

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Discussion Starter #5
You can take the belt off and run the engine, and see if the whine subsides.
How long can I run the engine with no belt? Doesn't the coolant pump rely on the belt?

I really need to get a mechanic's stethoscope. I assume that would let me hear any slight noise when I turn the pulleys by hand.

The engine is shifting at a high RPM because I was driving up a moderate hill during that video. I chose that hill to record the sound because I knew the high RPMs would cause the full range of its whine.

Thanks for the input.
 

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You should be able to run 5 minutes which is plenty of time. With cold engine, you will hear the whining of the air pump for two minutes, then it should quiet down for you to check.

Try turning the a/c off completely and see if it gets quieter prior to removing the belt.
 
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