Author Topic: Humming 6.9 bird  (Read 91 times)


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Humming 6.9 bird
« on: 15 February 2021, 04:40 AM »
Hello 6.9 gents,

I am writing to the group to ask if this is common or unique to mine, maybe hear of other owner's experiences.
My car has a humming frequency between 80 and 90 km/h which I find particularly annoying as I often drive on country roads in the speed range. It is not very loud but clearly noticable even with music on. Sounds like a bunch of bees in the trunk.
It was abundant ever since I bought the car in 2013.In the meanwhile she got a next exhaust system and a new prop shaft bearing, no change.
I'd say that the sound becomes more prominent with warm engine resp. after some time of driving, say 15 - 30 mins.
I wouldn't say that the sound has become notably worse over the years which makes me think that it may be typical for the engine or car.
The car has made ca. 10.000 km since 2013.

Any input is welcome.

   thanks ahead.

1976 Merc 6.9  since 2013
1974 Caddy CdV   since 1992  (for sale)


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Re: Humming 6.9 bird
« Reply #1 on: 15 February 2021, 07:53 AM »
I have the same humming sound as well, mine is at lower speed, approx 70 - 80 km/h, then it stops as I go faster.

I think it's from the diff.  I changed the oil a while ago but it made no change.  Maybe clutch pack wear?

I now Robert (rumb) rebuilt his diff,  maybe he has some advice?
1977 450 SEL 6.9 - Astralsilber


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Re: Humming 6.9 bird
« Reply #2 on: 15 February 2021, 01:08 PM »
My 6.9 has no odd noises at any speed. But this model is well known for eating its diff. I rebuilt mine because it was making terrible clicking and occasional grinding noises in slow corners. When taken apart it became clear that the diff had already been rebuilt before. Since then 15 years and many miles have passed - all good.