I have a slight whine on decel between 55-40 mph. (Stock 3.31 gears, long repair history on the gears - we won't go there). I notice that in the summer, it's barely there, but now that the weather is colder (below 50) it's much more noticable. Why is that? Does the gear oil get thicker and thus less effective? I thought thicker gear oil would actually reduce noise, not make it worse?

 

Your gear oil is multi viscosity (75W-140) so that should not be the issue. Are you sure you rear differential fluid level is up to snuff? And since you made mention of it. If your gears are not set up properly for some reason than the oil will make little difference.
Scott

 

Maybe the tolerances loosen up just that little bit when their cold . "No" whine becomes "slight" whine.

 

Well, this is my second differential - they replaced the whole shebang after 3 attempts to fix the original gear whine. So these gears are right from the factory, and they still whine a bit, but I aint letting them touch them again after that whole mess. It's not real bad, more like a flutter (like a loud pigeon in my trunk) then a whine. it's just weird that it's much louder in cold weather, barely noticable when warmer. It's definitely gear whine - immediately goes away when I give it even a little pedal.