What you describe sounds like your mid bass/woofer driver in the passenger door is bottoming out. They will make a loud pop or slap sound if you push the excursion too far. Are you cranking it when you hear this? Is your bass setting cranked?

If you are hearing this when you are not listening loudly, it could be that particular channel of the amp having a problem or still a problem with the driver itself. Not too hard to check... just swap the passenger driver with the one in the driver's door. If the sound follows the speaker, replace the speaker. If the sound stays on the passenger side, investigate the amp and wiring. My 02.

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