Traction Lock differential noise burst

While \\"driving along in my automobile\\" I have on three occasions heard a VERY loud buzz I believe is coming from the differential. This noise sounds a little like the normal slipping on snow clunk - clunk but at a much higher frequency and noise level.
I hope this is a track lock sensor problem, but don\\'t know how to trouble shoot it since it has only occurred three times in the last 6 months, Any thoughts?

thanks,