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Recently purchased 996 very loud intermittent tapping

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 I really need the help of somebody who is experienced with the 3.4 996 engine.  I purchased this car about a week ago, it has a very loud intermittent tap, unlikely it's the IMS as it's been upgraded to the LM bearing. one minute it'll start and purr like a kitten the next minute it'll sound like the engine is full of rocks. 


 No loss of power, drivability issues, smoke,  check engine light, oil consumption just the noise 


 These videos were taken five minutes apart?!?



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Sounds quite external to me. You may want to remove the aftermarket airbox and see if it's interfering with anything. Also check the entire belt and see if it's damaged anywhere. Then remove the belt and spin all the moving pulleys by hand (clockwise) and see if you can duplicate the tapping. 

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The rhythmn doesn't correspond to piston knock or valve train. Sounds too slow for those. The crank pulley spins about 12 revolutions per second at idle. Most of the pulleys are smaller so they spin even faster. Also there is some non uniform double tapping. Indeed a strange tapping.

Edited by Ahsai
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Oh man, that doesn't sound good at all. Have you checked if the clanking is comimg from one of the cats? You can place a screwdriver between the cat and your ear and try to hear the tapping. Another thing to check is if the spark plugs are all tight.

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1 minute ago, Twenty Clicks said:

 We have listened all over the engine, cannot isolate where the noise is coming from it appears to be coming from under the intake manifold 

It does sound like it's coming from there in the videos. However, there's really nothing rotational there other than the crankshaft after you removed the belt.

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One of the five timing chains may have extra slack due to the guide pads being worn out and coming into contact with something internal?  You don't have the noise at idle and it get louder at higher RPMs, making me think centrifugal forces are causing two components to come in contact.  Are there any aluminum particles in the oil filter?  How many miles on the engine?  It sounds louder than I would expect for a chain hitting the case or a cover and I would also expect the frequency to be higher (more taps per second).  Is the noise louder on the back of the engine?  Broken finger on the pressure plate possibly?  I would raise the car up and listen from underneath.  I would also remove the oil filter, cut it open and look for particles.  Worst case is that something inside the engine is making this noise and you will probably have particles in the oil filter and you are looking at a complete teardown to clean it out and rebuild.  By the way, the guide pads can be replaced with the engine in the car, but from what I have read, it is difficult.

Edited by Splitting Atoms
left out word. added sentence.
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Yup, that's the bank 2 scavenge pump. Can be removed (if enough space) without removing the cam cover. Hard to imagine it making such clanking noise though.


You can use a mechanic's stethoscope to pinpoint the source better.

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A couple of the posters thought the noise may be coming from the exhaust.  The catalytic converters are located behind the bumpers.  They do disintegrate internally when they get old, but the noise sounds too loud to be to be the exhaust.  But I would definitely take a closer listen to rule out the exhaust before I start opening up the engine.  Was the exhaust recently removed?  Could a bracket be lose hitting one of the pipes?

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