The rear end on my 99 (old body style) sierra 2500 2wd has been making noise ever since I've owned it But now its getting worse. It makes a howling noise that changes pitch the faster I go. I changed the oil in the diff a few months ago and it didn't seem to make a difference. I'm pretty sure It's a 14 bolt back there because it's massive. A friend told me that just because the diff has fluid doesn't mean that the axle tubes do. Do you think it's possible that the axle tubes are dry and the noise is metal on metal?