On my pickup, i have a whistle in the cab when i get goin about 75 (Depends on how fast the wind is blowing and what direction its coming from) I have a pretty good idea that it is comin through the firewall somewhere, ive checked the door seals. I tried taping off one section of the firewall where the fan gets it air from right up by the windshield, so that it would suck air from farther away and that did not work. Do you have any other ideas that i should try or maybe where the whistle is coming through at? Does this happen very often?