Pull your alternator, power steering, and A/C belt off and see if the noise continues.
If it is ENGINE speed related first eliminate the possibility of engine accessories. Then move to the driveshaft. A stethoscope may prove helpful if it's is able to be replicated without driving the car on the road.
If it is coming from the center of the car you can use a stethoscope with a friend to poke around on the floor and see where the noise comes from.
Alfas first, then everything else.