Has it always been that way?
When I was at the GenEdge Atlanta meet several years ago, one guy had that problem. It turned out, and I kid you not, that there was a MAGAZINE in his fan housing. Left there no doubt during assembly because there's NO WAY it got in there any other way.
We had to remove the fan from the housing (which can be done inside the engine compartment -- we did it in the WalMart parking lot), take out the problem stuff, then close it back up.
It was a Hot Rod magazine, as I recall.
Anyway, long story to say it could either be something caught in your fan throwing it off balance, or rubbing on it inside the housing.