You might want to visually inspect the speaker. They have a tendency to sometimes get caught causing the sound to be "weird". Anyway, there's a coaxial tweeter in the center of the 8" cone. The tweeter is mounted on a plastic thingy and the cone sometimes get's stuck on that, so if you can free it, and maybe lube it a bit? you might alleviate the "farting". I know mine did this, but don't remember if the symptom was "fart-like" or something else.

Good Luck!
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R.I.P. - Wrecked in Hit and Run --> http://www.stleasing.com/scty_boy.htm <-- '98 S70 T5-M ***** '00 C70 HPT 5-speed, NBM/Beige Full Leather, 400w Dolby PL