300te driveline sounds?

[silversleeves]

Symptoms:
Low speed sounds coming from under car (front I think) - sounds like one of those child's toy fisher price lawn mowers with the popping bouncing balls.
Noticeable at low speeds and varies consistently with speed. At higher speeds it seems to disappear.
Sound is audible in drive and neutral, braking and accelerating. At times the sound disappears. There is no difference to sound when brakes are applied.
There is no sound if drive is engaged while car is on a hoist.
The front flex disc at transfer case looks fine. Transfer case fluid level is fine, rear differential fluid is fine.
There is no support bearing on the front part of the driveshaft.
Front differential fluid level is fine.
Passenger and driver side axle shafts are recently new.
There are no pulsing feelings in the brakes when applied.
Tires are new.
No sensations in the steering wheel when sound is audible.

Possible diagnoses:
I am aware that my serpentine belt tensioner is on the way out - could this be the issue?
Wheel bearings?
Brake rotors? (pads are new)
Any suggestions?

Thanks for having a look?
Jamie