I don't want to scare you, but it sounds just like the problem I'm having. I like your description of the noise because that is what mine sounds like too. I've been looking for the culprit all winter and I have been thru the upper part of the motor, transmission, and wheel bearings to no avail.
The only thing I have not been into is the crankshaft and I have a feeling that is where my problem might be. I just noticed my low oil pressure last weekend. I can say the noise has gotten louder with time.
2009 FLTR MOP
"A Goverment should fear it's people not vice versa"