Buzzing in Subaru dash

Hello! Today while driving form CT to Maine my 2004 Legacy started making a buzzing sound. It was on the right of the dash near the corner of the windshield. It only happened once we went around 75-80 mph (Mass Pike, we were going with the traffic flow). I called the dealer since I got the car only a month ago. They said they’d have a hard time figuring it out since they can’t take it up to that speed. My husband thinks it might be the speedometer cable. Any thoughts?

oops! I meant the LEFT side of the dash.

I wonder if it could be the seal around the windshield. I had rental car, a Ford Taurus, that had a terrible buzz when I went above 60 mph. We returned the car to the rental agency and they gave me a free upgrade to a Jeep Grand Cherokee. When I returned the Jeep, the agent said that the problem was that the windshield had been replaced and the glass company had not sealed it properly.
You’ve only owned the Legacy for a month and this may be the first time you have had it above 70 mph. I doubt that the Legacy has a speedometer cable. I think the speedometer runs from an electronic signal from the speed control sensor.

Triedaq is correct that your car uses a speed sensor that sends an electronic signal. The days of those old noisy speedometer cables have been over for quite a few years.

In addition to a possible air leak at the base of the windshield, it is possible that you were hearing some harmonic vibration that may or not occur again. Sometimes harmonic vibrations required very specific circumstances–sometimes in combination–(specific type of pavement, specific ambient temperature, specific vehicle speed) in order to take place.