I ran into a problem with the FM transmitter for my iPod (a Belkin TuneCast Auto). For some reason, it was randomly turning off, resulting in a loud blast of static from the radio.

Turns out the problem was the "Sound Check" feature on the iPod. This feature attempts to adjust the volume level between songs so that the volume is consistent between songs. It appears that this feature messes up the other feature on my Belkin FM transmitter, which is that it turns itself off if the iPod is turned off or disconnected.

So, if your FM transmitter is mysteriously turning itself off, try turning off the Sound Check option on your iPod; you can find it under Settings.