Vague Buzz in Dash
 

I'm hearing this vague, muted buzz that seems to be coming from the dash/windshield area on the driver's side. It's one of those sounds that isn't distinct enough to pinpoint its location - it's just there.

It seems like it's more wind-related than road surface related since it increases with speed but is there regardless of the road surface. It is there on very smooth surfaces and doesn't get worse on rough surfaces. It's intermittent: bzzz...bz...bzzz..bbzzz...

It there regardless of engine speed or A/C fan speed. I checked the wiper blades and trim around the window and each seems secure. Side mirror feels solid. Thumping or pressing on dash doesn't have any effect.

Puzzled. Any similar experiences?