The only way to know for sure if it was caused by that incident is to isolate the problem. First, disconnect the amp and see if the speakers still make that noise. If they do, then you know it's not the amp causing the problem. If they still make the noise, try and connect the speakers to another radio source. This may be difficult as you may have to remove them. If you have a spare speaker lying around, fade the sound to the speaker making the noise and then disconnect it and connect the spare speaker up and see what happens. If the noise is still there, you will have to go from there.

Good luck.