Whistling sound after 30 minute driving

[Hoppus]

Hi,
I have issue about whistling sound while driving my car, Opel Astra Sport Tourer 2011. I bought this car about 4 months ago, and the sound start after 2 months.
We normally hear the sound after driving about 30 minute at 100km/h (60mile/h) and then slowing down to 50-70 km/h. The sound is very strong, I have recorded from inside and outside the car while driving via Youtube: https://www.youtube.com/watch?v=jSV32VlCh_E . The first garage has checked the brake, tyres and said it is safe, but they can't find the sound source.
We think the sound come from the middle bottom, and this sound like flute or maybe some metal touching, not sure...

Can anyone have idea where the sound come from and how to fix it?

Thanks in advance,
Hoppus

[MagicRat]

It sounds like a bad bearing on one of the engine's accessories. Most engines use a rubber belt on the front or one side of the engine. The crankshaft rotates the rubber belt and provides power to a number of accessories, such as the alternator, power-steering pump, coolant pump, air conditioning compressor etc.

The only accessories that make a high pitched squeal like this are the alternator and any idler pulleys or tensioner pulleys. Replacement of the offending item is the only repair.

Does this noise change in pitch as the engine revs change? A bad bearing like this will change the tone of the sound it makes as the engine spins faster or slower.

[Hoppus]

Hi,

thanks for the input!
Not very sure you mean about "the engine revs change", since I don't know much about car engine.
What I can think, the tone (frequency Hz)) doesn't change with the speed. When it happen at 100km/h and reduce gradually to 50km/h, then the sound strength also reduce. If the speed increase, then the sound comes louder again.
I will ask my garage to check on the bearer, but it seems coming from the bottom back side (passenger area).
Can you describe which alternator actually is? I need more specific position for checking.
Many thanks!

[vgames33]

The alternator is under the hood.

When the noise happens, kick the car into neutral and free rev the engine. See if the noise changes pitch this way. If it does not change, then it is not related to the engine.

Wikipedia shows several turbocharged options for your car. Its possible that you are hearing the turbocharger whining through the exhaust.

[Hoppus]

Hi,

the whistling sound seems to be gone right now. I did replacing the tyres two weeks ago from winter to summer type and I don't hear the sound anymore at the same condition as I hear it.
But I am still not sure how could this be related with the tyres, or maybe the cover (velg) of the tyres may be the reason.
Do you have some explanation for this?
Glad that this was not an engine issue

Thanks for the input,