it's the intake, if you have a aftermarket filter that's a open cone shape, you'll hear that noise, it doesn't have to be cold air, it can be hot air, i have a hot air, and mine does that, it's just that when the air is sucked in through the filter, it goes in a swirlin motion, and creates that whistlin sound, it's perfectly normal, no need to worry, just think of it as a turbo, and your spool is boostin up, hahahaha.