My 1985 300TD, with over 300,000 miles, makes strange noises when weather is cold. I should note that the entire car is in tip-top running condition....good compression, valve clearances always adjusted, recent top notch valve job, synthetic oil (rotella), new glow plugs, recent injectors, EGR valve disabled (ball bearing in rubber fitting), rebuilt vacuum pump..good vacuum, etc. etc.....but I'm wondering if the cold weather noises are possibly indicative of a problem.

The noises in question come from within the engine compartment just forward of the glove compartment....the noise happens only when the engine is cold, and is more pronounced when real cold (like this morning when temp. was -5F). It sounds like there is a manual gear box mounted right on the firewal where the mono valve is. At lower speeds (lower gear), the noise is more pronounced when I take my foot off the accelerator....sounds just like a loud manual gear box would sound if you were in first or second gear and you took your foot off the accelerator, allowing the engine to rev down while in gear. The noise definitely corresponds to engine RPMs....thus, I am assuming it is engine noise from the vicinity of the manifolds.

I dont recall when this noise started, for a while now. I may have become more pronounced after I cleaned all the gunk out of the exhaust manifold when I did the valve job (exhaust manifold more resonant??)

My question is this....is it just simply engine noise, and if so, why is it so pronounced when outside temps are so cold (I dont have a garage, so the car sits out in the cold all night). Does it have to do with the firing of the cylinders/combustion noises when cold?

Note: I also recently replaced all the coolant temp sensors and the like, but the noise still may be related to some sensor or vacuum related compensation device for cold starts. Anyone else have this experience, or know what is happening here??

Thanks,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)