Tires whistling maybe?
 

Gotta quick question.. So I've been noticing a continuous whistle sound while I'm driving... Not sure what it is and I've searched around the forums and can't pinpoint what exactly it is. I filled my tires up to the proper psi and I can still hear it. Any idea?

It could literally be anything, based on that description :)
Does it change with speed or with rpm?

look for something snagged under the car.