I had a real nasty rattle that developed a while ago...it would be real loud at idle but would disappear at higher speeds. I couldnt figure it out, and with loud engine noise, I couldnt pinpoint the exact source.

Then one day when I was removing the air cleaner housing, I found the blue plastic cap to the low pressure port for the AC system that I had lost a while ago. After removing the cap, I never heard the rattle again!!

THis is probably more common than you think because most of us set down bolts, sockets etc. in the little "tray" at the air intake and end up "losing" something.

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)