High pitch tone coming from tweeter?

Recently one of the Clarion 1" tweeters in my car has begun emitting a high pitch shrill tone whenever I turn the radio or CD player on. This system was installed by the previous owner of the car and before I begin to tear apart the install I would like to know if anyone has any idea what might cause something like this? The system has 3 seperate Alpine amps, 1 dedicated for a sub in the back and the other two running the 6 speakers up front. There is a capacitor inline somewhere but I haven't looked yet to see where. Most of the wiring and all three amps are underneath the carpet in the hatchback so I'm hoping someone will have some sort of suggestion before I have to start pulling things apart.

The only other info I can give is that the noise was kind of random when it first started but over the course of several months became more and more frequent. At this point, it is either there when I turn the system on or it starts within about 5 minutes of playing something. It seems like it starts sooner if I am playing the stereo very loud so I'm not positive it's the speaker, possibly the amp.

If any additional info is needed let me know and I'll try to find out, thanks in advance for any advice.

Follow Stevdart's advice. If the sound stays put then we know it is in the signal chain. Describe the sound for us, is it there when the vehicle is running? Does it go up and down with the engine RPM, do you have to put your ear to it to hear it or is it so damn annoying that you want to put your hammer through it? Is it there at all volume levels or does it dissappear as the volume level is increased?

Try a muting plug in the amp in question (take a el cheapo rca cable and cut the end off, leaving about 4" of wire. Tie the inner conductor to the outer conductor and plug this end into the amplifier. Test for noise again. If the sound is still there we have a problem either at the amp or in the wiring to the speakers. Check to see if the crossover for the tweeter is mounted anywhere near a computer or other power wire.

Switch the speaker wires over from side to side, does the sound now travel to the other side of the vehicle? Post up your results.