Why is one of my 08' Prius noticeably louder than the other

I have 2 '08 prii. One of them seems to now be somewhat noisier on the road than the other. The predominant noise I am hearing is one that modulates with speed of vehicle. There is a concentric component to the sound which makes me think its related to tires or worse case bearings. My guess is that the noise is due to something related to the tires. They both have OEM goodyear integrity tires. Quick inspection finds no large stones in the tire treads but will check more closely if you think this is possible cause. Here are the variables:

Vehicle A (increased road/tire noise) / Vehicle B (less noisy)
---------------------------------- --------------------

Mileage-------> 13,000-------------------- 3,200
Tire Rotations --> 2 ------------------------ 0
Alignments ------>0 ---------------------- -- 0
Equipment ------>#5 ------------------------#6

Special Notes: Vehicle A which is more noisy has had 2 tires rotations - performed at 2 different Toyota dealerships. On the latest rotation at 11K miles I observed that the work was not carefully performed as witnessed by lack of attention to tire pressures. The pressures were wrong after rotation - apparently who ever rotated the tires left the original pressures in the tires. I'm not even sure if they rotated them front to back or crossed them (should have paid closer attention to where I found the pressures but just was so pissed that I quickly adjusted them myself).


Questions:

1. could improper rotation (diagonal instead of front to back pattern) cause the integrity tires to 'sing' more than they do when new ?

2. could the 13000 miles on vehicle A versuses 3000 Miles on vehicle B's tires account for the additional noise ? Have you noticed the original tires becoming noisier with more miles on them ? Is this expected with 13000 miles or is it more likely after more mileage than 13K.

3. I don't think this is the case but is the #6 package better noise insulated inside than the package 5 ?

4. Could the hack that rotated my tires left a lug or two loose on my wheel ?

5. Could this be something worse like bearing ?

As usual - thanks in advance for your comments folks..