If they all do it did the one you test drove sound like it?
I had a new mk1 focus in 99 after a thousand miles if you revved it you could hear like a siren noise something to do with the turbo. Had a colleague with identical car that didn’t do it. So he parked next to me at ford dealer. Got manager to come put and revved my car. He says oh they all sound like that. I says we’ll his doesn’t and he starts and revs his.
Funny they stopped fobbing me off then and replaced the turbo