88 chevy k20 new style no vent window

Hi i have this problem a howling air noise when the wind blows over or truck is at about 40 mph and up , if you are on the highway and get right up close to a trailer truck or some cars the noise goes away i have taped off the grill all hood and door seems taken the tail gate off took the tool box out, the skid plate is missing so i made a piece of cardboard and tryed that, took the antenna off moved the mirrors, just can not seem to find it, can anyone help please email me [email protected] thanks.