Loud noise coming from left side axle?

I'm not sure what this is. Happened 6 times now off and on. First snow storm I was using 4WD alot so I thought it had something to do with that but I havn't turned it on in 2 weeks. Wife just said it happened twice on a straight away, at 50mph and at 30mph. Kind of felt like there was ice on the wheel well but there isn't any snow on the ground now. When you come to a stop, it stops. Then you can drive on for awhile and you'll hear it again. Very loud scraping sound from the left side like its on the axle or something? I've never seen anything so I'm wondering if my warranty is still valid. I think I have 100k warranty because i'm sure it won't be cheap if its an axle huh? Front brakes are fine but the told me I need back brakes. Don't think thats the issue either. For some reason I think the 4 wheel drive may be the issue? Have you had anyone complain of this type of thing? Thanks for your help.
Larry