What’s this noise?

manchu · 30-11-2023, 09:39 PM

Hi all.

MC diesel has just started making a new noise and I can’t pinpoint where it’s coming from.

170ish kkm. Drives fine, no warnings or faults.

Disappears sometimes. Increases with rpm.

https://imgur.com/gallery/3N3OhWv

Care to take a guess at what it is?

Cheers

**aussiblue** · 30-11-2023, 09:49 PM

The lost 10mm socket bouncing around in the valve cover. But seriously, check the oil fill cap is tight and see if the noise is louder with the oil fill cap temporarily off at idle. Also check the dual mass flywheel is not coming apart (per https://www.youtube.com/watch?v=yO2RpLrd48c) and the alternator mount is not lose (per https://www.youtube.com/watch?v=H9TDHevZgQU). Or even timing chain per https://www.youtube.com/watch?v=7W5AfL6uCyg . Or even diesel knock from a failed injector per https://www.youtube.com/watch?v=TcQ4wK8yncA and https://www.youtube.com/watch?v=-f-Xy6MOlK0 and https://www.youtube.com/shorts/Vi_xb_NYFmw etc

manchu · 30-11-2023, 10:17 PM

Quote:

Originally Posted by aussiblue

The lost 10mm socket bouncing around in the valve cover. But seriously a0 check the oil fill cap is tight and see if the noise is louder with the oil fill cap temporarily off at idle.

Checked with cap off and no difference.

It’s very much that kind of intermittent rattle though. Seems to come and go a bit depending on incline/decline. It’s only had standard maintenance (other than transmission repair 8yrs ago) so very little chance of loosing a socket etc inside somewhere with moving parts

Shortly before I first heard it today I’d been doing a bit of ‘spirited’ driving. Nothing crazy, just some sharp turns trying to get my boys in the back seat to bump into each other, but more crazy than normal so if there was a lost socket somewhere that might have moved it around.

Ps. Makes the same noise when fan is off so not that.

manchu · 30-11-2023, 10:56 PM

Quote:

Originally Posted by aussiblue

Also check the dual mass flywheel is not coming apart

Prime suspect I think. All these sound very similar and noise seems to be concentrated in the that area (while also soundin like it’s everywhere).

https://youtu.be/EI1m5D7x9MI?si=0-MTh3cllLq2Jwtm

https://youtu.be/y0jpTAURMZY?si=NfJsWv_xLHVFiYli

https://youtu.be/JcCQz937uwg?si=5Y5BjM1PCrjmPg1s

Now the dilemma of whether it’s worth fixing considering it’s a hail damaged write off that I bought back from insurance for $2k nearly 4 yrs ago. I’m tipping it’ll be almost that or more to fix.

manchu · 11-12-2023, 11:08 AM

Well the noise was gone the next morning and hasn't returned so either that missing 10mm socket has found a nice corner to rest in or the problem only rears its head when the car is given a mild flogging.

I did speak to my mechanic and he agrees its probably the flywheel. Ballpark price of $1.5-$2K to fix, so if problem comes back I'll be new (used) car shopping instead. It's just not worth putting that sort of money into it.

AlCan · 08-05-2025, 08:12 PM

Old thread, but maybe the same thing as I had.

My troublesome teen (Mondeo), the one that's got the 2nd gear downchange problem, has a very smooth sounding engine. Unlike another I know, it doesn't really have any of that diesel combustion knock that some do.

During one of those missed 2nd changedowns when it wouldn't change up again for the prescribed time (15? 30? Seconds), I had to keep going in first gear, and had to rev it quite hard for a while to get out of the way.

After that, it developed the diesel knock I've mentioned. I thought Bugger, must have given the injectors too much of a hiding, or something. Anyway, it carried on like that for the rest of the trip. But like yours, next day, the noise was gone again. As if it's a temporary adaptation or something, in response to being over-revved?

I wonder if this has anything to do with the noise you've had?

Another car has a distinctive knock when it's cold. First time I heard it, thought a big end bearing was going/gone. But after warmup, the noise has gone. I now suspect it's either injector or glow plug problem.

Dunno about yours though.

AlCan · 09-05-2025, 10:35 AM

Uhhh. Not over-revved, just high-revved.

NZ XR6 · 10-05-2025, 01:33 AM

The diesel noise from my MB has always been quite variable. It seems sensitive to fuel variations and runs quieter on Gull diesel than than the stuff now imported through Marsden Point since the refinery was closed. It is very quiet at 110 plus, even on some of the climbs on the Waikato Expressway - smooth and quiet at 130, obviously a European vehicle!

It's noisiest on cold mornings until warmed up.

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