Why was my radio antenna vibrating?

So a little background: I work near a man made brine lake that is a by product of the borax and mineral mining that goes on here. I rescue birds from the water and rehabilitate them. I drive around a lot because the lakes are very large. I got out of my truck today and the antenna from the radio was vibrating and making a buzzing sound. The truck was off, the radio was off and the lights were off. Nothing was on. Could there be a magnetic field causing this? It's worth it to note that there was a lot of lightning today and so I thought maybe lightning was about to strike my car, or something. I've never noticed this before and I go to this spot often. It is right near the waters' edge. Could there be something radioactive in the water producing these effects? It wouldn't surprise me....the water is pretty nasty and it's pumped up from very deep under ground.