Humming after 150 km/h

[danils]

Hi all,

My Zed, that is 23i automatic, has a problem I can not understand. Since some months as long as I reach 150 km/h, I start suddenly hearing a low humming noise. This noise has not a precise localization, it seems that all the car resonate

I also feel a slight vibration at the steering wheel.

I had a visit to the tire shop today that checked the balance of all the wheels and they are ok!

What do you think about?

Many thanks!!

Daniele

[Dr Stig 2]

Originally Posted by danils

Hi all,

My Zed, that is 23i automatic, has a problem I can not understand. Since some months as long as I reach 150 km/h, I start suddenly hearing a low humming noise. This noise has not a precise localization, it seems that all the car resonate

I also feel a slight vibration at the steering wheel.

I had a visit to the tire shop today that checked the balance of all the wheels and they are ok!

What do you think about?

Many thanks!!

Daniele

A few things.

A) 150kph - I am jealous. We lose our license on the spot anywhere in Australia if we are caught at 150kph (and we can have our cars impounded in some states)
B) Is the noise louder with the top up and windows closed or when you have the top down (and how loud is it)?
C) Does it get louder on different road surfaces (smooth / rough).
D) Can your passenger hear it as well (they can often move around the cabin to try and locate the source)?
E) Is it a low pitched hum or a high pitched hum - and does it change in pitch depending on speed (or does it appear at one speed, and dissappear if you change your speed slightly? These are what we would call a harmonic vibration)

[danils]

Hi!

A)We have to pay attention as well but fortunately we have some straight motorways spot where you can unleash your Zed
B)I experience the noise with top up and closed windows.
C)It is the same on all surfaces
D)The passenger hears it as well but he/she can not locate it.
E)It is a low pitched hum and it does not change pitch with the speed. It begins suddenly when the speedometer reaches 150 km/h and it remains the same accelerating. As long as the speed falls below 150 km/h it suddenly disappears.

Thank you for kind reply!

[danils]

Forgot to say that it is not motor rpm related because I tried to reach the same rpms at lower gears and the hum does not appear. It is speed related.

Many thanks

[Dr Stig 2]

Not being there to diagnose it exactly - it sounds to me like a loose component vibrating - and not a tyre issue.

The problem with lower pitched noises is they can seem to come from anywhere as they dont attenuate as fast with distance as a high pitched hum.

The good news it though that lower pitched hums come from larger objects vibrating, not smaller ones. (Think longer strings on a harp - they have the lowest tone).

There are some iPhone apps you can get for free that measure volume - "librarian" is one. It has a trigger that sounds an alarm when you exceed an adjustable volume threshold.
I would try using it to measure the hum as you move the phone around the cabin and see if it is louder in one spot.

That way you can narrow down the search for what might be vibrating.
Put pressure on any nearby surface (that can either hold it still or change its effective mass so it vibrates at a different frequency) to see if you can make it go away to confirm you are identifying the root cause.

If you want help - just send a return ticket to Australia for me... I've never been to Italy.

[danils]

Thank you Dr.Stig you were very kind!

I will visit the dealer next week, I have spoken yesterday with a BMW mechanic who supposed a loose component, also because my car undertook the replacement of the main shaft and other defective parts at 8000 km. Maybe something loose after mounting the parts.

Greetings!!

[Dr Stig 2]

My pleasure.
Hope the dealer can fix it for you.

My local one has been great at finding obscure rattles and squeaks for me.

Our hot dry climate means every six months they have to service the roof mechanism to adjust it and lubricate.