Toyota Repair: 2003 Toyota Seqouia Radio pops, toyota seqouia, fm modulator

Question
Since we bought our 2003 Seqouia, we've been experiencing a weird feedback/popping noise thoughout the stereo speakers. The radio appears to be on with stations displayed, however there is no volume control and the popping is extremely loud. The noise doesn't happen continuously, it comes and goes. Which is why it's been hard for the dealership to troubleshoot the problem. I've taken the vehicle in and the dealership mechanics are all lost. The battery has been changed, the computer has been changed, the FM modulator has been changed. I tried to adjust the radio volume while the loud feedback/popping noise was occurring and I did notice I can slightly hear the selected radio station while the loud feedback is happening, however the volume is max'd out to do so. Adjusting the radio volume has no effect on the noise. Turning off power to the radio has no effect either. The noise just comes and goes as it pleases, with no specific interval when it occurs. Revving the engine up doesn't change the intensity of the noise. Your thoughts?  

Answer
I don't understand why this is hard for your dealer to diagnose, there is a sevice bulletin from toyota that addresses this condition(TSB AU001-04) take it back and tell them to look up this bulletin, it involves replacing the audio amplifier, it is covered under the warranty.