700r4 whines while coasting...

Hey guys, I just picked up a new project vehicle that I had a few questions about.
First off, It's a 1988 K5 Blazer with 5.7L and a 700r4 trans with np208. I picked it up off of a buddy as a winter beater for now with plans to turn it into another trail/hunting truck.
Anyway, I was driving it home and noticed a whine coming from the trans at 55-40 mph when I was coasting. Got home and checked the fluid level, and all was fine. No burnt smell, however the fluid was a little dark. When driving around it shifts really firm, almost as if it has had a shift kit put in, and has no noise while under load.
I was thinking about doing a fluid and filter change in the trans, but wondered if the whine was anything I should look into deeper/worry about. This is the first Automatic trans I've owned in quite a few years, so I'm a little rusty on the details.

Thanks for your time,
Nasty