It could be the motor mount ( at least it was in my case)
Jon

Barry-

That's why I asked how long he owned the car, as these odometers can tend to find themselves askew of "genuine" miles.....not that that's the case, but it was worth trying to ferret out new owner/new (2u) owner type info....

If you are indeed in KY, I might be able to look at it, schedule permitting.

Otherwise, it could be a loose air filter mount. My rubber isolators broke apart, and made a very loud noise outside the car, like bolts in a dryer, and sounded very low like rod knock from inside the car

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Current:
*1984 300D ~200K,1989 MR2 250K, 2012 Ford Fusion 4.5K (fiance's car to replace the uber-unsafe cavalier)
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Past: 1998 Chevy Cavalier ,2005 Saab 9-3 2.0T, 1996 Mazda Protege, 1974 Porsche 914 2.0, 1997 Land Rover Discovery 4.0L/5Speed, 1995 Jeep Wrangler

Top of the engine all checked out.
Oil drained. Mechanic who is tip-notch, and has serviced this car always for the 3 years I've owned it, will check out the bottom end.

Suggestions what it may be?

Flex disc?

Other?

Did he remove and check the vacuum pump?

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

Does it do while the car is moving, or will it do it while sitting still? Most drive line and rear axle noise will be rhythmic and change with speed, or noises happen during sudden changes in acceleration. Make sure to look at the motor mounts, and the shields on top of the motor mounts. I had a bent on that was rubbing on the body and making screeching noises like belts. Also look at the transmission mount. Could be a look exhaust heat shield, or even the exhaust rattling on something.

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In a situation like this, a YouTube clip is worth a thousand words of problem description.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Before I took it in, I could drive north of 25-ish mph and the awful rattle would quiet. But as soon as I began slowing down, loud again.

The above shounds a lot like the something loose on the Air Filter or Bracket.

OK the Mechanic going to look at something but where did He say He though the sound was comming from.
Also while He can look and see if there is anything in the Pan there is not too much of the bottem end he can get at from the small Oil Pan.

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84 300D, 82 Volvo 244Gl Diesel

New air brackets look great.
New turbo parts look great.
Timing chain, valves, everything on top etc look great.
New motor mounts look great.
1.5 quarts low on oil but not remotely catastrophic.
Mechanic going through bottom end tonite and tomorrow.

Flexdisk?

It's TOUGH to diagnose noises and rattles on the internet. The most likely thing would be the vac pump roller. If the sound is coming from that area I would NOT run the engine any more until I removed and inspected the vac pump. This would be a quick job. Those rollers have DESTROYED engines. If inspection of the pump does indeed indicate that the roller is gone, don't just replace the pump. FIND THE ROLLER and get it out of there.

I have had them get pretty noisy with the roller loose, but still attached. If that's what you find, then it's a quick fix.

Is the mechanic familiar with the breed?

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

Did he look at the harmonic balancer and the main crankshaft pulley? In addition to the vacuum pump, those things are known failures which make rattling noises.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Barry and the others who said Flex Disk, you are correct. Although under the car, sounds like from the front. I gave my mechanic a hug when he told me it's not the end of the world (eg, thrown rod), just a cracked flex disk.

Now that the problem has been run down.

I have an interesting one for you. Concerning my naturally aspired 1977 123 300d.. Came out of the cottage one morning. Started it up and heard a very bad sounding rattle.

A word of warning here. Any time you hear a rattle under the hood do not lean over the radiator until you have checked with the engine off that the fan assembley is still intact.

I have cerrtain knowledge that the fan let loose and killed a guy in my neighbourhood many years ago where I grew up. He was simply investigating the source of the underhood noise.

That said once I had quickly checked my fan I started the car again and heard a sound like the flex plate was perhaps the issue. A broken crank kind of makes a clang on startup but does not usually stay noisy once the engine is running.

I was just thinking this is great. Then the noise stopped and never occured again. Not that I am complaining but for the life of me that seemed very abnormal.

It did not sound much like the starter had not totally disengaged but in all likely hood just may have been randomly touching the ring gear on the flexplate. Not much else could account for what I experienced that morning. That it never reoccured again after that second shut down and start is what kind of cinched it for me. More a rattling type of sound than anything I thought.

I think it was one of my very early posts years ago when I posted a thread about it but recieved no reponses. It was only later that I personally concluded there was no other possibility that was logical. I did post that it seemed at the rear of the engine and low on my original thread.

I also have to wonder how the mechanic responded to your hug. They in general do not get that many.