Strange noise coming from cold start x5

[X5fan2075]

Hi

I have had my x5 for a month now. Since I have bought it I have had a strange noise coming when I start it cold. It’s like a ringing sound in the background and goes once engine is warm, I have had the chain looked at as to whether it is stretched and it was confirmed the chain is fine. The mechanic couldn’t pin point why it is making this noise but said it shouldn’t cause any concern as the cars vitals appear fine. Does anyone else have this noise? I have attached a video link:

https://youtube.com/shorts/lnFYyzIaN...jO0Z3BQveTHG1P

Car is a 2015 x5 30d m sport ran 73k miles.