Rear end noise

I am thinking about trying to fix the whine in my 8.75. It is a 489 case with 3.23 SG. It has had a bad whine while "floating" at around 40 MPH. The further you get from 40 the less you notice it. I've had the car about 7000 miles and it did it when I got it and hasn’t gotten worse. I did check if the pinion nut was loose and it isn’t. The only other rear end noise I have dealt with was an old Suburban 10 bolt. It made noise decelerating and I found the pinion nut was finger tight. I put locktite on it and snugged it and it was fine. I am familiar with the crush collar design of the 489. Any educated guess what could be causing it? My first guess was a loose pinion but if that isn’t it then maybe a pinion bearing?

Gear lap might be off a little. That can make them whine when there isn't a load on the rear end. That's just a guess though.