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I have a 2013 Subaru Impreza with a 2.0 liter engine and a manual transmission, and when I let out the clutch too fast while in reverse or 1st, sometimes the engine RPM will drop so low that it makes a nightmarish knocking sound before recovering. What is that dreadful noise? ( is it the transmission? the engine mounts? the piston grinding away at something god forbid? )

I'm familiar with the knocking caused by air being too hot and/or the fuel mixture being too lean, but to the best of my knowledge that happens under power and usually not with the engine is simply idling.

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Don't do that. The best thing, relatively speaking, that can happen if you do this a lot is that you will blow your head gasket. The worst thing that can happen is that you ruin your clutch or damage your transmission. –  Juann Strauss Mar 20 at 9:31

2 Answers 2

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It sounds like to me this is the general noise an engine makes when being "lugged" or run at a low RPM under heavy load ... basically, it makes this noise because there is a big boom in the cylinder and no place for the energy to go. This really is not good for your engine at all. You should try to be a little more gentle on your engine and drive train.

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"More gentle" in this case means release the clutch slower or use more revs. –  SimpleSimon Mar 25 at 11:23
    
@SimpleSimon ... Yes - Or in other words slip the clutch a little more. Give the engine a chance to gain more RPMs before allowing the clutch to engage all the way. You never see this happening to a car with an automatic transmission. –  Paulster2 Mar 25 at 12:15

That's the engine "Lugging".

Basically, the expansion is still happening in the cylinder, and it is doing one of two things:

1) The crank is being violently cranked forwards. This is usually when you get the 'pulsing' feeling in the car.

2) The expansion is happening, but the piston isn't moving at the right time.

It is not good for any part of your engine, transmission, or differential. You should avoid it.

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