Hi Jiho,
Be careful when diagnosing this, as sounds travel from front to back and vice versa.
I changed a front Universal Joint on my Cavalier as the noise was coming from the front footwell.....but
that Didnt fix it !!
So decided to live with a faulty gearbox.....wrong again !!
2 weeks later drone from rear suggested rear wheel bearings, so swapped both with used /nearly new bearings....and voila ..all was sorted .....sound was travelling to the front through the chassis.
On a more positive note, i have had 2 cars with droning back ends, which i eventually fixed ( 72 ford cortina + 69 Ford Falcon)
Both were rear wheel drive, and both had worn out hardy spicer couplings on the back of the propshaft.
I would suggest that you check the rear U.J. on the propshaft, as it runs dryer than the front one, and the bearings dry up and dont run properly. on all rear wheel drives.
swapping prop shaft with a good second hand one may sort it !!...and its easy to do .
I wouldn,t suspect the diff, as it would/could clunk as well as drone.
difficult to diagnose without hearing but wheel bearings could do the same, but check prop shaft first, then bearings i would suggest, especially with the V6 which supplies a lot of torque through the prop shaft (single bearing that drives the whole lot. )
let us know how you get on
klickster.