[Bruckner]

My car developped an annoying metalic rattle on cold startup, it fades away as the engine warms up. I tried to locate with a stetoscope without success, it seems to be coming from everywhere and nowhere at the same time. Any ideas?

 

<a class="" href="https://youtu.be/HRfltNCHK_U">https://youtu.be/HRfltNCHK_U</a>

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[MCL968]

Wow, that is not a happy sound! I had something similar to that years ago and it was the rotor contacting the distributor cap.

 

It sounds to me like it is outside the motor, so maybe alternator bearing or AC compressor pulley bearing?

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[tejon007]

Wild guess...broken AC mount/bracket (or some other mounting bracket)? Try putting the AC on and does the sound change?



Continue with the stethoscope, you should be able to track it down, even if it's inside the motor.



Best wishes.

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[dlearl476]

Sounds to me like one of your belt roller bearings is dry. Pull the inspection cap and put your stethoscope in there. 

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[94SilverCab]

Heat shield?

 

Jay

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[Bruckner]

Thanks everyone.



I temporarily ruled out the distributor cap and alternator since they're

easy to reach with the stethoscope and nothing obvious is heard.



The AC is hard to reach from the top, it hasn't worked in years so it doesn't kick in when switching on. But it's one of the most likely causes I think. I've seen this reported on the 944 boards.



Heat shield. That's a good idea. The one under the oil filter is easy to inspect, is there other ones?



I will look closer at the belt covers to see what I can take out to gain access to some rollers.

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[Bruckner]

I realize that the sound file I made with my cell phone distorts the noise, I also had a camera running and it's as close to it gets to the real thing.

 

<a class="" href="https://youtu.be/NwpCDxbZOl8">https://youtu.be/NwpCDxbZOl8</a>

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[dlearl476]

There’s a little rubber plug that is in the top of the belt cover right behind the distributor. 

       

 

You should be able to just put the end of the scope on the timing cover and tell if it’s coming from inside word of warning, it will be LOUD.

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[ds968]

Heat shield rattle is a very common issue ( I’ve had it in one of the 968s ) but at least from the audio clip, your rattle sounds totally different and much louder than I recall mine to have been .  Then again depending on how bad the separation is, it may make different sounds . Definitely worth checking . 

 

IF the sound is coming from your drive belts ( timing & balance ) case, shut off the engine immediately and don’t turn it back on until the culprit component ( rollers , etc. ) is replaced.  If they seize while the engine is running the same catastrophic result can happen as your timing belt braking .  

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[dlearl476]

I realize that the sound file I made with my cell phone distorts the noise, I also had a camera running and it's as close to it gets to the real thing.

 

https://youtu.be/NwpCDxbZOl8

I’m 90% sure that’s a pulley. But I have to say, it sounds an awful lot like when my water pump was dying, too. The difference with my WP was that I got belt squeal when it first started up, too.  

 

 

FWIW, two belts ago, I was concerned with a noise like that (only MUCH LESS) and I asked the tech who did my belts about it. He told me “a little whine is normal when cold, but it should go away by the time you reach operating temperature. It’s when there’s no whine you should worry about your belt being stretched”

 

But your whine, like my water pump, is way louder than “normal.” I caused my own misery, because I thought since my belts/pulley/water pump were 6 years, but only 4,000 miles old, I’d just replace my belts. False economy. I had to do it all again after my water pump failed and just about broke my timing belt. 

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[Bruckner]

The mystery is solved : it was the balancer belt that was way too loose. At fast idle speed (cold) it was slapping against the water pump pulley producing a weird harmonic effect, the slightest increase in RPM would straighten the belt enough to clear the pulley and kill the sound. The belt was so loose the balancer gear could be turned at least half a tooth by hand. We also noticed that as the engine reached its hot idle speed, the belt was still flapping all over the place but not making any noise. The tightening adjustment isn’t exactly a simple procedure but the result is conclusive, I had the car rest for the night and cold start was nice and quiet.

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[ds968]

Go out a buy a lottery ticket ; if loose balance belts come off enough to tangle with the timing belt …catastrophe happens !  

The  recommendation is to retighten them after the first 2k-2.5k after installation and then at minimum every 24 K miles but preferably 12 K miles intervals .  I must admit I’m very guilty of not observing that recommendation ( after the first 2.5 k miles adjustment , that is ,.) 

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[tejon007]

Bruckner, great you fixed it!



We just went through the process of cam belt and balance shaft belt replacement a few months ago (again). The BS belt tension is easier to set with a tool by Arnn Worx (not sure they're still in business, bought their tensioning tool several years ago). I'm sure the factory tool works fine too, but last time I looked it was $1,200.



Here's one version: https://rennlist.com/forums/parts-market...-once.html



One other point, even if the BS belt tension is correct, the timing of the balance shafts is very critical or you'll have lots of vibration. Supposedly, the timing mark you line up at the top rear of the motor by the bellhousing is the way to align the shafts (not the camshaft timing marks). It's also a little tricky because the BS sprockets have two keyways and the alignment is different for the top one versus the bottom one.

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[Bruckner]

That's interesting. Admittedly my method was suboptimal because of limited time to sort it out, we used timing marks on the BS and camshaft drive gears to line up and tension. At first we thought the top BS was a tooth off, but with correct tension seemed about right.



The motor spins very nicely but produces annoying tremors at idle, this despite new genuine mounts installed last year. I suppose less than perfect BS alignment could be the culprit.



At some point I will have to go back in there and do a more thorough job. In the meantime, no more scary noises on startup!

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[tejon007]

Well good luck with it. When I replaced the cam and BS belts, I also replaced the BS sprockets (and all others) as I didn't know if it had been done before...at least I had never replaced them before. I initially got the lower BS sprocket located on the wrong keyway, so it was like 180 degrees off. The result was noticeable vibration, so if you get it wrong, you should definitely notice the problem.



Maybe one more point. After tensioning the BS belt, turn the motor over via the crankshaft bolt at least two full revolutions and check the BS timing.

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[Bruckner]

May I ask you to describe the vibration issues you had? Was it across the rev range? How was the idle?



Yes I had a look at Clark's garage description of the confusing keyways on the lower BS sprocket.



I'm fairly certain mine are lined up, off idle the engine is perfectly smooth. However I may have overlooked the adjustment of the BS belt idler, I was more concerned with tension of belt and alignment of timing marks, the 0.5mm and 0.1mm clearance gaps I didn't even check. I will go back in there but first I'll have to order the correct tools (angle wrench and BS gear lock wrench).

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[tejon007]

Well, the motor is a high compression high performance motor. It normally has a noticeable "lumpiness" (technical term) at idle, so that does cause some vibration at idle.



The vibration with incorrect lower BS alignment - - I would describe as more than before even at idle and increasing with RPMs. It did not smooth out.



Tension the BS by rotating the tensioning sprocket clockwise.



And, yes, set the gaps correctly on the idler. But, it's really just there to prevent/limit belt slap. And, the BS belt isn't as critical as the cam belt. In other words, even if you run without a BS belt, you likely won't destroy your motor...at least for a limited amount of run time. Setting everything up correctly is a little tricky the first time you do it, but once you get it right, you should be good for years of driving.

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[Waylander]

Could just be the tappets draining of oil if you have used to thin stuff

They can take 20 mins to shut up

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