300SDL engine noise, need help...

Hello group.. We have a 1987 300SDL, that just turned over 250K over the holiday weekend..As far as I know, the engine has never had major overhaul but when
we bought it our mechanic said it looked like the timing chain tensioner was new as it had this shiny knob thing sticking out on the pass side of the front of the engine..The car runs flawlessly and uses very little oil but this weekend we went to north Texas and it was a bit chilly (28 degreesF) and both mornings, we started the car and it fired right up and ran good..But within about 45 seconds the engine started making a noise I can best describe as what an AC compressor sounds like when it's going out..Sorta made a grumbling, rattling sound..I raised the hood and it sounded like it was coming from the valvetrain area..Soon it stopped and didn't reappear till the next morning..But now we are home and it's not that cold it didn't do it..Could the timing chain tensioner be hanging up or something? I have a note into my mechanic but I think he is still out of town... Thanks in advance!!! John
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John wrote:

eliminate this first.
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I have an '84 300TD. I have heard similar noises (Loud, senseless racket) that turned out to be failed mounting bushings on the air cleaner housing. At slow idle it was the worst and it would vanish at 100 rpm faster than that. The noise is impressive; probably the loudest noise coming from under the hood. I could also describe it as shaking a paint can with a handful of rocks in it. Maybe the lower temperatures changed the idle speed just enough to set up the harmonic rattling. It was at its worst when the engine is idling slowly enough that is visibly shaking from side to side. Next time just lean on the air cleaner housing with some authority and see if the noise changes. Or you can lift up on the air cleaner housing. If it raises more than the slightest fraction of an inch, it's the bushings.
Regards,
Ken

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Hi Ken, Thanks for the reply...It's definately coming from the front of the engine, but so far has not done it again..The six cylinder diesel engine doesn't have the problem with the air cleaner bracket you describe, as it's remotely mounted on the inner fender..I do know what you mean tho, had a 300SD that did that and I had to replace the bracket...Had one guy mention the vacuum pump and I do believe it to be original..Somewhere I read an article that these engines are notorious for failing vacuum pumps and I right away talked to our mechanic about it and he said he had never experienced one fail on this engine..Been going to take the car out to him but won't do much good if it won't re appear...Maybe when it gets really cold here.. Thanks!!! John

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