I have read through the thread but have not seen the answer to the question: what does cause the burble and gurgle? So I'll take a stab: As far as I know, the flames are unburned fuel getting into the exhaust and igniting/burning in the exhaust instead, resulting in popping/burbling noises. This is more present with high powered turbo engines due to running rich and the fact that the turbine housing and wheel get nice and hot and help to ignite the unburnt fuel. I had a friend with a Lotus Esprit turbo, carburetor + turbo= big flames and lots of popping on deceleration. I also once owned a car with a engine with carburetor myself, that car was running rich, lots of popping and burbling. At some point the popping turned into big explosions in the exhaust, getting big enough to blow a hole in the middle muffler :) the muffler just split open like a coke can that had a lit firecracker inserted...

The other reason for burble is air being sucked into the engine, through the exhaust and exhaust valves on deceleration when the throttle is closed. Because there is no ignition and expansion of fuel and air on the 3rd stroke (or power stroke) there will be a slight vacuum in the cylinder when the piston reaches the end of the stroke. So when the exhaust valves open (to let the not present exhaust gasses out), air from the exhaust will be sucked in, resulting in a popping/burbling sound.

This is all AFAIK, please correct me if I'm wrong...

Sounds right JB1, the same burble noise happens with a co-workers Mustang and it is unburned fuel, but once the system warms up it goes away.

I use to have an 07 STi....I do miss that car sometimes. In a perfect world were I had more money I would own my Z and a STi

Interesting that it goes away when the car warms up. With the carburetor cars I referred to it became worse (or better ;) ) when the systems warmed up. Especially the Lotus, that really came alive when warmed up and driven "in anger"...

Bingo.:driving:

Sometimes I wish the Z had more "burble" too. Someone I know owns a r32 with an aftermarket exhaust and it sound awesome.
Kind of like this but not as loud.

Nice Lamo doors... Ah well, guess it's OK as VW owns Lamborghini...

BTW, that's loud... :)