TAKE IT BACK TO THE DEALER.....for stuff that you can't really describe in words over the internet, take it back to your dealer if you have a concern (or don't trust what people are telling you). You bought the car from them, take it back and ask questions.
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It could be BOV
It could be a bad clutch
It could be your still on the gas (for a moment) when you push in the clutch
It could be your muffler bearings
It could be that voice in your head telling you that you should have bought a SRT-4
I'm not ragging on you, I just trying to prove a point....everyone will tell you it's one thing or another (and it could be any one of those things or none of them)...and without experiencing your problem in your car personally, no one can be absolutely sure what you are experiencing. Soo, if you dont' know anyone who can tell you for sure what your problem is, take it to the professionals who were trained to diagnose the problems with our cars....LET MAZDA DECIDE....this is what you have a warranty for.
[/rant]