Chirp/whistle on cold start

[CalumDaly]

https://imgur.com/a/VsQdHk2

Seat Leon 2012 1.6tdi chirp/whislting sound on a cold start that goes away/dramatically reduces when engine heats up.
For context I changed my alternator with a second hand one maybe a month ago but the sound just started. Any ideas? Thanks

 

[Rakhsh]

I have a 1.4tsi and mine does this but it goes away as the rpm drops, not when the engine heats up. I'd be interested to know if this isn't normal, I always assumed it was normal.