Whistle in rear passenger area

[wvinson]

I've had a whistle in my rear passenger area for a while -- I've had the cab to bed seal replaced as well as the cab vents replaced.
I've read on the forums somewhere that there was a bolt missing underneath the seat where you could see the ground. Anyone have a similar issue. It's not the tea kettle noise coming from the dash on the interstate. This is at most speeds with any window down or the sunroof open. Thanks!