I have been trying to get to the root of this problem in my 93 245 off and on for a few years and could really use some advice. When there is a cross wind, or when I'm passed by an 18 wheeler, AND I'm doing around 70 mph, I get a party horn sound. The sound comes from under the dash, near the cowl area. The higher the cross wind, the louder the sound. Closing the vents or turning on/off the A/C has no effect. I'm convinced that the cross wind causes this as it does't occur when it's calm, even up to 80 mph, and getting passed by a truck can causes it, just as I feel the slight rocking of the car from the cross wind. If there is a strong cross wind, it can happen as low as 60 mph. I taped over the cowl vents to see if the air rushing across them caused it (something vibrating inside the HVAC system like a saxaphone reed?) and it had no effect. I recently had the dash out while replacing the blower motor and thought I'd see about addressing this issue. I made sure all the ducts were firmly attached, and even insulated them with the 2" black sticky-on-one-side pipe insulation (great stuff!) but it didn't help (I drew the line at pulling the evaporator/heater core).

Anyone else have this problem or have any advice? Thanks.