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[opelman]

Hi
I have an old opel omega b 2.5v6, which has got really bad in the drive. it is a howl sound and it comes at approx. 40km / h. at 70km / h it gets almost gone, then it gets up by approx. 80km / h.
I have changed wheel bearings at the back. There is no surrender if one turns. the sound is constant.
what could it be? 

[BazaJT]

Differential? Although I don't know if the sound would alter when turning or not.

[dave the builder]

This needs methodical diagnosis, and more information needed , what work has been done,how many miles/Km has the car done  auto or manual box 
could be many things, diff noise is very possible
also prop center bearing, wheels out of balance, worn front or rear suspension (out of line) is there un-even tyre wear 
clutch and flywheel issue, sticking brakes  is unlikely at such speed
can't rule out things like a window slightly open, wheel arch plastic liner or trim loose (resonates with speed) exhaust issue 
lots of possibilities 

[Entwood]

When I was trying to diagnose a strange drive train noise I jacked the car up at the back onto very solid axle stands, then jacked the wheels to "level" using the lower shock absorber mounts, then ran the engine in gear. This enabled the drive train to function in "normal" aligenment, and it became very obvious that my problem was the diff. Far easier way of checking than trying to stick a head out of a window and listen !!   

[olm]

Check the center bearing

[Doctor Gollum]

Prop bearing is pretty robust, even if abused. The diff however will wail like a scorned banshee if all the oil has dripped out over the last 15-20 years...

[Nick W]

Prop bearing is pretty robust, even if abused. The diff however will wail like a scorned banshee if all the oil has dripped out over the last 15-20 years...

I 'topped up' my diff when I had the car jacked for new rear shocks. It took a litre of hypoid   and was much quieter for it.

[opelman]

Hello again
I can tell a little about the car, it's a 1996 Opel omega B 2.5v6 estate, with manual gearbox that has gone 485000 km. I have, as I said, changed the wheel bearings behind, I have also changed the oil on the diff. The diff is a LSD diff.

[Auto Addict]

Air vent blocked? Right hand Rear wheel arch inside boot?

[opelman]

Hi again
I've figured out the noise, it was worn pinion stock.