When does this noise happen? at idle? at highway speeds? at maybe just a few thousand RPMs over idle?
I would think if it were a idler pulley or any pulley problem, you would hear that with the engine running with the hood up right away. if not, putting a load on the motor (driving basically) might cause the pullies to whine IF they were infact the problem.
I personally might try leaving the e-brake on, and slowly start attempting to drive the car in gear, to put a slight load on the motor (as if it were driving) and see if a friend outside of the car (or yourself) can suddenly hear the pullies whine. Sometimes pullies only whine when there is a load on the motor, so a side from taking the pullies off and inspecting them, they can be annoying to diagnose.
But there are easier ways to diagnose a pulley problem.
Here is what I would do 1st: Perhaps try sitting in front of the motor with the car running, and blip the throttle body cable, see if the increase in RPM from doing that causing the pulleys to whine in front of you.
Also try putting a screwdriver around the idler pulley base (basically not on the spinning pulley, but near it), listen through the screwdriver (your ear on the handle) for any rumbling noises. Obviously you leave the car running while you listen through the screwdriver. Be sure not to slip if you do this though!
How many miles do you have on the car? How about mileage on the mods? Did it rain recently or did you wash your car? Perhaps it was just water buildup on the pulleys causing them to whine (not an uncommon problem, due to the vents in our cobra hoods). When its been dry for days, does the car still whine when you drive it?
Honestly, my guestimation is that its just water/condensation build up on the pullies from either washing or weather related. And it will probably go away if it were dried.
