Anybody ever have a problem with their windshield wipers (or something nearby) buzzing/chattering when at highway speeds?

The goofy thing is, it goes away if there's moisture (i.e. I hit the squirters). But as soon as it dries out it chatters again.

I've tried:

1) replacing the blades (twice)
2) Putting electrical tape over the vents
3) Putting electrical tape over the rubber windshield molding
4) Putting electrical tape over the squirter nozzle assemblies
5) Removing the wiper blade completely

Right now I'm thinking maybe the arm tensioner spring is vibrating?