wet fart slapping noise.

Hey all, I've recently bought a stock 2008 Subaru RS 2.0, and have fitted a full exhaust system and CAI.

I have a 3' Cannon attached, the sound is loud and it DID sound good, but now for some reason in 2 or 3rd gear I get a slapping / wet fart noise coming through the muffler.

I went to the exhaust place, no brackets or anything were out of place, and people have said that 3' cannon are too big and has decreased the back pressure...

All your opinions would be appreciated.
Thanks

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