I have a 2006 GP GXP and I am having issue with speaker. Every time I hit a bump I hear hissing from speaker until next time I hit a bump. I've taken it to the dealer a few times they say they cannot find a short. They said that it was the amp in the back. They replaced it and for about 2 months the speaker was fine now the noise is back.

Any suggestions/ideas as to what it may be?