There could be a couple of reasons for this.
First of all, you're on to something. Large, high-flow exhaust systems often burble or pop on decel because of the lack of back pressure. High engine speed creates high manifold vacuum, which is increased even more when you let off of the throttle and the butterfly valve in the throttle body snaps shut. This creates a low pressure condition in the exhaust: a small amount of air going in, a small amount of exhaust coming out. The weak exhaust pulses aren't able to overcome the difference in barometric pressure between the outside, ambient air and the intake manifold, causing a reversal of exhaust flow, pulling cool air into the exhaust system. When the cool, outside air and the hot exhaust mix, the colder air rapidly expands and creates a burble. It's the exact same event that occurs when ambient air around lightning quickly expands, and produces what we know as thunder, just smaller. Having large, high-flow pipes facilitates this exchange due to the reduced velocity of the exhaust pulses, which translates directly to a reduction of back pressure.
In addition, some manufactures tune cars to burble on decel. When the throttle is cut, the ecu injects a small amount of fuel and greatly retards the engine timing. This allows the spark plug to fire while the exhaust valve is open, creating the burble sound. I honestly have no idea if this applies to GM or the LS7, but I'm sure someone on this forum will know something about it!
I hope this helps, please feel free to clarify or expound on anything. I'm still learning about LS7's (I'm new to the Z/28 world), so I won't be too much of a help with solutions. I've read somewhere that in 2011, GM actually made an effort to accentuate the burbles and pops in LS7's because they found out that most customers actually enjoyed it. I, admittedly, am one of those people!
