I'm not sure which heading to put this under, so excuse me if this is in the wrong category...

I've noticed in the last week or so a "whistling" inside the cab of my truck. It doesn't seem related to windows or doors or anything like that. I've also turned the heater fan off and this has had no effect. It's always the same single tone that grows in volume, and usually it's at ( on my gauges anyways) 1200 RPM @ 102Kph ( with the A/T 285's on the truck I'm not sure how much the speedo is out). Also it seem to be only under load...I was coming to work today and it did it while climbing a sllight incline in the road, but didn't do it at the same RPM and speed on the exit from the bridge I was crossing. When I hear the sound a bit more or less throttle and it's gone. It also seems louder in the middle of the front seat towards the dash, but since I'm usually alone when this happens it's hard to bounce around the cab and try and locate it.

I'm not 100% sure, but I think it only occurs on cold days.. it was -12 C this morning.

I don't even know where to start looking.

Thanks for the read
Steve
'00 Lariat 4x4
'97 Mystique LS
'94 Mustang 5.0 Convertable