Quote:
Originally Posted by Noggie
I'll chime in here as I have some insight to this.
I have heard this sound on my M5 too, only when cold, but, the sound was also there to an extent all the time, I heard this when driving with windows down and had a wall on one side of the car.
BMW is right, this is perfectly normal, nothing to worry about.
Dwaynos is also right.
I immediately recognized this sound when I heard it on my own car as air in hydraulic fluid being forced through a tight orifice.
Just so you know that I'm not pulling something out of my ass, I am an Aircraft tech with 10 years work experience on commercial aircrafts, for the past 14 years I have worked in the oil industry. I have commissioned a vast number of big heavy hydraulic equipment on many on/offshore oil rigs. I currently work in management with responsibilities for upgrades on Drilling rigs.
Yeah, I'm getting old 
Ask any hydraulics tech about this sound and he will say that this is caused by oil with trapped air passing through a tight orifice, in this case a filter.
Think about it, you car has sat for a while and the oil has drained to the bottom of the oil sump and is cold.
You start up the car and start circulating this cold oil around the gearbox, the low viscous oil is trapping air inside it from the drained areas of the gearbox, air that cannot easily escape from the cold oil. As it is forced through the filter at high pressure it makes this sound.
As the oil heats up the air will escape the oil much easier, and by that time the gearbox is filled with oil and pick up very little air.
Hence why the sound disappears after a while. |
Hey Thank you for the detailed description. I also think that it will be the way you described it.