"Marble in the can" sound source?

[MrDangerUS]

Re: 87, 944 N/A, 2.5 SOHC
Just replaced the belts (like 2-3 weeks ago), and car was running fine.

Recently, I drove the car to my summer cottage, 260 miles, using expressway, stopping only for gas, almost non stop.
Everything seemed going fine.

Next day, in the morning, I started the car and heard irregular "marbles in the can" type sound, non-cyclical, when idling, (car not moving, gearshift in neutral). All belts are new and properly tensioned, AFAIC.

When I increased rpm, the sound got faster and somewhat more resembling chirping and lesser in volume.
Using a mechanic's stethoscope, I listened to all components, including oil pan, clutch housing, exhaust and cat. Only the water pump had some whining instead of low frequency humm.
When I took car for a test ride, the sound "disappeared" at higher rpm/speeds, or perhaps, got lost in the background noise.
Any thoughts?

[jep32]

Did you replace the water pump while doing the belt?

[rhett]

I think this might be what I have mentioned to a couple of forum members recently. I had this exact problem. It turns out that although the belt appears nice and tight around the WP and through the spring tensioner, once it goes over the cam sprocket the drop down to the crank sprocket can be loose. The belt then flaps around and hits 'stuff' and a 'marble in a can' sound is heard intermittently. It does generally go away once the engine warms up as the block expands and the slack is taken up. You will need to re-tension the belt, making sure that uniform tension is applied from the spring tensioner after the slack is taken up from the other side first.

260 miles is just enough for a new belt to slacken up after fitting. You always need to check these after a while to compensate for this.

Take the belt cover off and watch the cam belt as it exits from the sprocket cover, if it appears to be vibrating this might be where the problem is. Always keep your fingers away from this area when the car is running!

[MrDangerUS]

No, I did not replace the H2O pump.

Belt loose?
Well, that makes sense, and you 're right, the sound is much less pronounced when engine warms up!
But, my car (87 and all 87+ cars) has a spring loaded tensioner...
? Shouldn't it take care of the belt tension?

[rhett]

There is a common misconception that the spring tensioner compensates for belt tension because it has a spring. What it does is applies a known preload tension to the belt, then once this has been applied you lock the tensioner in place. The belt is then under static tension and no further movement of the tensioner roller takes place. If the belt then 'settles' (and they all usually do) the belt will become slacker. However, nothing is going to compensate for this except slackening off the tensioner fixings, allowing it to re-apply the preload and relocking it down.

The design of this engine also makes it easy to have a belt that is 'tight' on the WP side as the tensioner does its job, then slacker on the side that exits from the cam sprocket. When tensioning the belt you have to ensure the tension is equal if you do not want the belt to 'flap around'. To do this involves very small movements of the belts and sprockets just prior to tightening down the tensioner. There are several threads about tensioning belts with the spring-tensioner in this forum..have a search..most of it is good stuff - it is what I have used.

[kdollar]

Did you replace roller bearings when you put new belts on?
My 951 had a bad bearing and made a similar noise on cold start.
Turned out it had a bad bearing.

[MrDangerUS]

Yes, I have replaced all rollers/idlers, except bal.shaft sprockets and tensioner.

Thank you for a good advice. I found more info here:
Timing belt?

[rhett]

Yep, that thread pretty much covers everything, as the tech bulletin says, do not rotate the engine using the crank pulley while the tensioner is loose. However, to ensure the tension is even I do gently wiggle the cam sprocket counter-clockwise before locking down the tensioner. This tightens the belt on its drop down to the crank and ensures the tensioner applies a more even pressure. It is a tiny adjustment but it has worked well for me. Once everything is locked down, then you can rotate the engine a good few times to check tension around the WP and on the drop side. You want to be able to twist the belt 90degs at a point opposite the WP pulley on the drop side. Any more than that and it will flap, significantly less than that, and you may have your belt too tight.

[MrDangerUS]

"Marble sound" = deep du-du, for sure. Park your car and tear into it, immediately!
This time, I was very lucky.
The lower balance shaft belt tensioner bearing disintegrated at the exactly right moment. In my driveway, that is.
What is puzzling, I lost all my balls! Did not find even a single one, anywhere.

The PN# on the part indicates 944-102-217-05, all steel part.
The closest I could find on the net is made of plastic, PN# 944 102 025 07.
Can I use this one? Porsche 924 944 968 Tension Roller for Balance Shaft Belt Toothed Gear INA | eBay

[alan6272]

could it be coming from the cat? is the sound for sure in the front of the motor?

[MrDangerUS]

It was not cat!
It was the tensioner bearing falling apart! See pictures in #9.

[Gawernator]

the part you showed in the link is 944 102 025 07

[Gawernator]

It looks the same though..

[rhett]

Well saved! Crikey you were lucky there, glad you got it sorted before major damage occurred. I guess the moral of the story is change all the rollers..

[CHICKS]

Good catch, mine made the exact noise, under the same circumstances, but ended up being the balance shaft idler roller bearings were toast. Good luck!