I am hearing a whirring noise that seems to be coming from the rear

I noticed it last night, and think the symptoms are more noticeable now that the weather is turning cooler and I have to keep the windows rolled up.

It really makes the noise when I'm in first gear, or coasting in neutral., but not as noticeable in other gears.

If it is the diff, do you think it's better to just get one from a junkyard or for sale on the net, or have a rebuild done at a tranny shop?

Have you checked the fluid level? Does it change or go away after the car warms up or when you take a turn? Just trying to get more info.

My only experience with a "bad" diff was on my old '84 Monte Carlo SS. I over doubled the power, blocked off the 1st-2nd accumulator and the first time I punched it after breaking it in it ripped darn near all the teeth off the spider gears. I replaced it with a junkyard 3.73:1 posi because I had very little money left and it worked fine. I don't normally suggest running junkyard parts but I did have good luck. Of course that was a solid axle car too.

My 2 cents

Nate