This could be caused from a whole array of things.
Assuming your input signal to the amp is clean, and your installation is good, my first guess would be a bad output transistor. Most car amplifiers are Class A/B, which in a nutshell means they use two transistors for each output channel (or more in a bigger amp, but always a multiple of two). The transistors take turns with the load. When one is on, the other is off, and they hand off the signal. If one is bad, or partially bad, the other may still be working which will generate a signal out of the amp, but it will be heavily distorted. Sometimes depending on how the transistor is dead, the output will sound like audio mixed with static.
This is just one guess at many possible issues your amp may have.