S320 loud speaker static
 

I'm currently driving my Dad's S320 and going nuts because while listening to music, any time I hit a turn signal, extremely loud static blasts out of the driver's side rear door speaker. It stops when I shut the stereo off, and when I turn the stereo back on, it's still gone. But it will come back with the turn signal, and after a while of driving, the static starts coming on all the time. It always goes away when I shut off the stereo.
It's not a subtle sound, it startles the heck out of me and you can't hear the music over it.
I've read in searches that the amps on this car are often the problem when it comes to speaker static, rather than the speaker itself. Does that sound consistent with what I'm experiencing? Any way I can check other than spending money on a new amp or to have it rebuilt? Assuming all the connections look OK.
As a side note, if it's relevant, moving the fader to all front or just the other side does not make the static stop.

Am I reading correctly that the static occurs in only one speaker?

I am pretty sure it is just the one in the rear driver's side door, from moving my head to each speaker. Another note: volume of the stereo does not affect volume of static.