So I removed my front passenger door panel to isolate a small rattle I'd get when turning up the volume on my stereo. Well low and behold my curiosity and thoughts were spot on. The previous owner upgraded all four stock speakers to carbon fiber Polk Audio MOMO speakers. No wonder my stereo sounds very clean with little to no distortion. Btw I fixed the rattle which was a loose clear plastic cover that was unglued to the regulator frame structure. SCORE!!