Humming / roaring noise only at 50km/h, disappear at other speed.

[Lafetoo]

Hello all,
I have noticed my 2015 W212 250 CDI ( only 72000KM) facelife having the kind of humming or roaring noise when speed at 50 km/h only, other speed +- 1 km/h noise gone, for the pass 3000 km , I tried very hard to pin point this issue without success, I changed front wheel bearings, transmission mount, both 4 tyres were changed 8000km ago.
The noise only appears at 50 km/h, no matter in D or N.
Anyone can input any advice or similar experience will be highly appreciated

 

[keith of kent]

It would cost nothing if you have something to support the car to run it in drive with the rear wheels off the ground. By doing this last week on my 2015 212 we discovered that the rear wheels were out of shape.
Currently being refurbished

 

[Lafetoo]

Thanks for you input Keith, I have the rear wheel check and rebalanced 1000km ago, so this may rule out the out of round issue.
Thinking if I have bad quality tyres, even though they are still considerable new.

 

[Bellow]

Thanks for you input Keith, I have the rear wheel check and rebalanced 1000km ago, so this may rule out the out of round issue.

'May'.
It can be balanced and still be out of round.
What is odd here is the very narrow and specific speed band it occurs in.

 

[keith of kent]

"'May'.
It can be balanced and still be out of round."
My tyres were all recently balanced, Michelin PS4's not very old but the wheels were out of round.

With the assistance of a passenger with good ears it should be possible to determine whether the noise is coming from the front or the back of the car
If you go on and off the gas at that speed does the noise change? Presumably you have loaded/unloaded wheel bearings by swerving gently left to right?
Does braking gently have any effect? With the car on a downhill at 50kph put it in N (NOT D ! ) and alter the engine revs. Any difference?
Do you by any chance have a friend with an E class who would let you briefly try their wheels on your car as that would definitely rule the wheels/tyres in or out of the equation?

 

[Jonesy161]

When I first read your post, I immediately thought of my experience with the intercooler hose being split. Just a thought?

 

[Lafetoo]

"'May'.
It can be balanced and still be out of round."
My tyres were all recently balanced, Michelin PS4's not very old but the wheels were out of round.

With the assistance of a passenger with good ears it should be possible to determine whether the noise is coming from the front or the back of the car
If you go on and off the gas at that speed does the noise change? Presumably you have loaded/unloaded wheel bearings by swerving gently left to right?
Does braking gently have any effect? With the car on a downhill at 50kph put it in N (NOT D ! ) and alter the engine revs. Any difference?
Do you by any chance have a friend with an E class who would let you briefly try their wheels on your car as that would definitely rule the wheels/tyres in or out of the equation?

I thought about the borrowing wheel testing idea but I don't have friend with mercedes wheels .

 

[Flyinspanner]

To check for any deformity to the wheels, a dial test gauge is perfect, you can check for run out on inner and outer faces, check for flat spots (and with the wheel off you can do same for brake discs etc - though I don’t think it’s part of the issue here)
about £20 for a cheap one on eBay.

Jonesys idea above is good too you can check hoses with a smoke test/ washing up liquid/ or ‘Easy Start’ spray on induction hoses.

 

[Lafetoo]

The humming is from front right ( driver side), checked both front caliper and brake pads no findings.

 

[Bellow]

Going slightly leftfield.... try altering the tyre pressure. If it is a tyre resonance issue, the speed at which it occurs should change with a different pressure.

Don't discount the possibility of it being something non-wheel/tyrre/suspension related and the sound path makes it appear so. I spent an age trying to pin down a rattle on the rear brakes of a W123 that was the front brakes and a rattle at idle on a smart that sounded engine mounting related but was the warning triangle rattling in its plastic box behind the passenger seat.

 

[Lafetoo]

Thank you all for your valuable advises, I finally found the problem source is a slightly out of round front left tyre, using spare tyre to test on all 4 corners one by one.