I have installed a new amp a few weeks ago and just noticed that only when the radio is off I get the annoying whine coming from my rear speakers. The amp isn't powered on when I get the whine either which is really wierd. When the radio is on (volume set to 0) everything is totally quiet. So what would be causing this? I"m stumped..

mine was do that i found it to be the alternator going bad

the power wire to the amp might be near the speaker wire that is feeding the back speakers causing this. I have seen other systems that have been setup with the power wire and speaker wire too close together and you get Crazy feedback. this sound is only noticible when there is no sound out of the speakers right??

right.
the ampt is mounted right below the 6x9'* mounted on a trunk "wall". The speaker wires are only about 2-3 feet long and don't run by the power wires.

you may want to consider moving that amp if you want to get rid of that speaker noise

Also consider twisting the pairs of speaker wires. Telephone and network cables are done like this to minimize or even fully protect against stray noise. You can also use a "ground loop isolator" which plugs into the rca inputs of your amp in case you are picking up noise that way too.

If you have time for some troubleshooting, do this:

Unplug your RCAs to the amp, then turn on the car. If you still have an alternator whine, then your internal noise filter in the amp is probably shot OR your wires to the 6x9s is picking it up from the power cable. If it goes away, then your RCA cables are picking up the noise...upgrade to RCAs that are shielded or get that ground loop isolator I talked about, or try running the RCAs and the power cable on opposite sides of the car.

Hope this helps ya.