My car is an '83 240D. It started making a howling noise recently, and I made an attempt to record it this morning in the hopes that maybe someone here could diagnose it.

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'83 Mercedes 240D -- What is this noise?? - YouTube

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1983 240D, 272K+ miles (odo broke)

It's hard to hear the howling over the belts. I'd fix that then see if you still get the howling noise. The howling could be the speedo cable. Try removing the instrument cluster and drop a few drops of lube down the cable. In the past I've removed the trans end as well and spun the cable many times until I can see the lube starting to drip out the other end. Clean up any large drips of lube and make sure it's not coming out in volume, then reinstall everything.

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I'm needing to replace a burnt-out bulb in my instrument cluster anyway, so I'll have my husband take a look at the cable then. And while he has it out, maybe I could get him to fix my odometer as well, haha (it only works when it's cold outside).

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1983 240D, 272K+ miles (odo broke)

ODO fix is quite simple.
WAAAAAY easier than the dieselgiant.com fix makes it out to be.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Well, that's a relief to know! The dieselgiant.com fix looks friggin' scary!

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1983 240D, 272K+ miles (odo broke)