at first, I thought it was the tires, but now I know it's not. Perhaps a bearing? What could cause such a thing? 85 300D sedan

Usually wheel bearings howl or sound like a jet engine. One way to test it is to slightly tighten the adjusting wheel bearing collar/nut thing on the front wheels to see if the sound changes. If the sound doesn't change it may be something else.

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Worn differential will also howl.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I had a high pitched howl in my wagon and really thought it was probably the differental and was prepared to live with it,while replacing rear calipers i discovered that my right rear hub bearing was bad replaced the bearing and was happy to discover the noise was gone.
I would normally associate a bad wheel bearing with a lower pitch growling/rumbling type noise so this one fooled me.It is a tough job to do without the proper tools,no fun at all!
Don

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Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles

Front bearings flex discs carrier bearings

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

Brake caliper sticking slightly can cause that also. Does the noise go away when light braking is applied?

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84 300SD was driver,now donor for 89
89 420SEL diesel swap
84 300TD parts car,soon to scrap

there is a bearing on the center of the drive shaft also. and a u-joint that can howl.

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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!

I was/am chasing the same noise. I had already changed the front wheel bearings, no help. When I changed the motor mounts it got better. When I changed the center supprt bearing for the drive shaft it got a lot better. I am sure the noise I still have is from the engine compartment.

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1987 300D 146K Original #14 Head
One eye on the road, the other on the temp. gauge

Gone But Not Forgotten
1983 380 SEL Long gone
1983 300SD Wish I hadn't traded it in on a 90' Corvette
1989 300SE My all time favorite
1995 E420 Went like a bullet