When you say a semi passes you, do you mean going in the opposite direction so that you are driving into the turbulent air coming off his trailer? I'm thinking it might actually be the door frames. Our doors have the window frame wrap around the front of the A-pillar. A stiff headwind can get under and I believe it lifts the frame off the gasket. I was working in North Dakota one winter, had to drive from Fargo to Grand Forks straight into a bad headwind. At 75 mph or more it was so loud in my truck that I couldn't hold a phone call. My truck is not overly noisy either, no weird noises or anything, stock exhaust, moderate A/T tires. Normally I can knock out the miles in comfort with the cruise set at 80. But headwinds, it get can get noisy and I don't suspect the windshield. I'm thinking it's the door frames, at least on mine. That isn't to say you don't have an air leak around your windshield though, just something to consider.
