Drivetrain noise when in gear

[rlyoung]

It may have been there all along, but I'm guessing not. It's a very faint "whoop" "whoop" noise which is rpm dependent, and is--I think--coming from the middle or rear of the van. If I'm motoring along on a quiet night below highway speeds I can hear it, but it stops if I shift into neutral and coast at the same speed. I'm guessing this eliminates the tires, but I wonder if the drive shaft turns when I'm coasting with my foot off the gas? I haven't yet checked for play in the drive shaft.

2001 AWD stock Astro

[Captn. Crunch]

It could be a multitude of things. First off, yes your driveshaft will be turning. If your wheels are turning your driveshaft is turning. It seems your noise is load dependent. As in engine torque applied to your drivetrain dependent. Engine torque will induce loads on u joints, transmission bearings and so on that may cause your noise but only when under load. Good luck

[rlyoung]

Thanks. Maybe I'll start with the wheel bearings. Also, is it ever done that one starts a vehicle on a lift, tires in the air, and puts it in gear and accelerates to locate sounds?

[Captn. Crunch]

Yes you can run a vehicle on a lift. Make sure it’s set well, make sure there is someone competent in the cab and make sure you pay very close attention to your surroundings while under a vehicle with rotating parts.

[rlyoung]

Thanks.

[Mmusicman]

Engine torque will induce loads on u joints...

^^ This would be my first guess.
Does the condition change when you go in and out of torque-converter lockup?