Whistling Sound?!

[true blue blood]

I notice when I start my car in the morning there is a slight whistling noise? MAybe its my imaginary blower in my head. I thought it might be an exhaust leak but I thought exhaust leaks were a rattling noise like bb's in a can? I haven't checked to see if its because I have my ac on or fan going making the whistling/ suckin sound! Any ideas? Thanks