[RFI] Strong, broadband noise during wet conditions

[Matt]

I'm having a problem with very strong, very broad noise that seems to be 
associated with wet conditions.  We recently had two significant 
snowfalls, and of course everything was very wet as the snow melted.  
The noise I have is just a constant grinding (I can't think of a better 
term to describe it) that is virtually continuous from 40m up through 
10m.  It is worst on 40-20m.  The snow is all melted now and things have 
dried out a bit, and my noise level is back down to normal.  The 
connection to the moisture is a subjective evaluation, but I've observed 
the high noise level during wet conditions, which then abated upon 
drying, on at least two occasions.

I had a friend come by with a mobile HF installation to see if he could 
hear it in my driveway, and perhaps isolate it to something inside the 
house.  Not only could he hear it in the driveway, but he could also 
hear it 1.5 miles away as he drove home!

My initial thought is something power line related, but I thought I'd 
bounce it off of the RFI experts on here.  I'm not sure what else could 
produce such strong noise over such a wide area.

Thoughts?

Matt, N0EYE