I just noticed last night that there's a distinct rattling sound audible through the open sunroof, almost like marbles rattling in a can. So far, all I know is that this sound is coming from the driver side front of the engine compartment, almost as if from either the power steering pump or the AC compressor. I'll probably do more troubleshooting tonight, but thought I'd pick the wise minds on this board before I start. The only thing I have planned is just to check the PS fluid level. Any suggestions on what else?

Probably going to have to narrow it down with the hood up and engine running before anyone will be able to help. Which car (Model & Year)?

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Steve
'87 300TD - 132K - Soon 4-Sale
'84 300D Turbo - 122K - Driving
'77 VW Type II - 77K - Restored
'08 250EX Ninja
English Bulldog (Brier) - My best friend. Passed away 12/02/04 while in my arms.

83 300TD

I checked briefly last night with the hood open and engine running. I'm 90% sure it's coming from the vicinity of the PS pump and AC compressor. What are symptoms of either failing? My first thought was that one of the bearings on either component was heading to the grave.

Also checked for loose parts vibrating against each other, or the fan brushing against something - didn't find anything. AC has been out for some time now- my suspicion is that it just needs a freon charge (there's been a known leak for a while- compressor will still engage if you jumper across the freon pressure switch).

Don't forget to check the AC bracket mounting bolts. There are three of them. Two of mine broke and rattled in the bracket making a noise similar to your description.
Richard

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Make Biodiesel
Not War

'85 300TDT

Check your A/C belt. See if it hasn't dropped some chunks. Mine made the very same noise when that happened. Does it quiet down when A/C is off and get bad when its engaged?

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

AC pulley bearing at the clutch will usually rattle most when the compressor is not engaged and less when the compressor engages. Worth checking the bearing there as it is not an uncommon failure.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

I'll be sure to check the belts and mounting bolts. The compressor hasn't been engaged in over a year because of low freon, so I can't compare the sound with the AC on and off.

Invest $10 in a mechanic's stethoscope. DON'T touch any moving parts with the probe or your ears will ring for an hour.

If you've never used one before. You will be amazed.

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You can get farther with a smile and a gun than you can with just a smile.

1984 300D
1985 300CD
1980 300SD
1993 SL500

One thing you can do to isolate the source of noise is to use a long 3/8 extension bar (or similar item). Put on end of the bar on the suspected component. Put the other end in your better ear. The bar will transmit the sound like a stethoscope. Just watch out for moving parts like belts, pulleys and fans.

Or better yet, if you know any nurses, invite one over ask her to bring her stethoscope.

What about the vacuum pump?

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

Well if you have a R4 compressor, my bets on it. After 2 rebuilts that had the same marbles in a can sound within a year of installation, no more rebuilts for me.

you need to isolate the sound. The vaccum pump can also make a rattle but it is more like a regular loud ticking like a loose lifter.

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85 300D 305K miles WVO(105K so far), Mine
82 300SD 235K miles BIO-D, Wifes
96 F250 SC PSD 4X4 285K MILES BIO-D, Ours
89 BRONCO 182K MILES sold
Finally Compression Ignition Only!

My 300CD has an inoperative AC system. The compressor is shot and the belt is disconnected. Shortly after I bought it I noticed the same kind of sound.

The compressor had worked loose in its bracket. Made a heck of a racket. I'd check that first. If you idle the engine and shine a bright light down on the AC compressor you'll be able to see if its loose.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.

I had a sound just like that on my SD, the compressor/A/C don't work anymore, but it still had a belt (in poor shape) and whenever the engine was at around 1,200-1,300 rpm I'd get this rattling noise, it happens to be the rpm at 25-30mph so it was quite annoying in the city. I checked and checked and finally figured out it was the clutch plate on the front of the A/C compressor vibrating at that particular speed. I cut the belt off, and the sound is gone!! Someday I hope to have A/C......thats a day a looong ways away.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I finally traced the rattling sound- it's coming from a plate on the front of the AC compressor clutch. There's what looks like a ring on the clutch facing the front of the car, rattles pretty bad at around 2,000rpm.

So, who knows what this rattling plate is, and how it can be fixed?