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Need some help identifying this weird noise that only comes right after I turn off the engine for a 04' MDX 120k miles.

Normally after turning off the engine, all should be quiet after a second. I have now noticed some belt slipping noise after turning off the engine. The engine does not continue to crank after ignition is off, so it is not dieseling.

I don't understand what could be continuing to rotate, after the engine is turned off.

18 months ago I had the timing belt replaced, as well as the A/C compressor. Not sure if the timing belt pulley / tensioner were replaced properly at the same time. I have recently noticed another noise coming from the drive belt, and it turns out the belt tensioner had a gritty tension. So I replaced the belt tensioner last weekend. While replacing the tensioner, I noticed one side of the drive belt seems to be slightly glazed. Upon inspection of the A/C compressor, the pulley of the A/C compressor does not entirely align with the rest of alternator, p/s pulleys. It is off by perhaps 1/4".

The new belt tensioner is bouncing a bit consistently while engine is idling. Not sure if that is normal... Any ideas appreciated. :confused:
 

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Update: Turned off A/C before turning off the engine. The slipping scratching noise went away. Not sure if it's the belt slipping or A/C clutch... to be continued...
 

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Upd: turned off AC Before turning off engine. engine bay still made that noise. Sounded like a hissing, depressurelizing sound. This is truly weird.
 

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I also hear this weird rattling noise whenever I turn my car off for a couple of seconds and then it stops. I have a 05 mdx 93k miles. No idea what it is, haven't really paid much mind to it.
 

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When the ignition is first turned on, before engine cranks, there is a charging noise. I am guessing that is the fuel system pressurizing.

When engine is turned off, there is a slippage/discharge noise... could be A/C compressor blow off valve (not sure the 04 has one, but I know some say Odyssey 05 does). It's too hot in Vegas to not use A/C but I will keep my ear open for it...
 

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Upd: turned off AC Before turning off engine. engine bay still made that noise. Sounded like a hissing, depressurelizing sound. This is truly weird.
That's completely normal for an AC system. During AC operation, the compressor compresses the refrigerant from low pressure gas to high pressure gas, then the condenser condenses that high pressure gas into liquid. The flashing of that liquid through the expansion valve (essentially a throttling process) will cause your car to cool down through the evaporator.

Because of that, there is always some part of the refrigerant that remains liquid in your system during AC operations. Once you turn off the car and the AC, that liquid will evaporate on its own, and you will hear hissing sounds. Perfectly normal.
 

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Thx, havn't heard that noise on any other Honda's I have. When the AC compressor was replaced I did run a couple of weeks with an over charged AC system done by the Mech. The clutch was constantly cycling until I brought it back and he discharged the pressure.
 

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Annual update. The noise continues, hot or cold, AC on or off.

A friend said it could be due to my K&N air filter. I will place in a paper filter and try again.
 

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See if the noise changes when you press on the brakes, just after turning the key off. I think you might have a vacuum line that is possibly leaking.

Is it louder from inside the car or outside?
 

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Discussion Starter #10
Wow, that is the most helpful insight so far, thanks!

I have noticed:
Engine off, hissing noise, 3x times.
Engine off, brakes on, no hissing noise, 2 times
Engine off, brakes on, hissing noise, a pause, and another hissing noise

So it does appear there is a relation.

I thought the ODB II would indicate a vacuum leak, but perhaps that is not available on Honda's?
 
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