First off, it's no problem and completely harmless.What I'm a little concerned about is that now I'm occasionally getting a slight pop on decel (engine brake or downshift). It doesn't happen all the time, and it's not that bad, but I was just concerned that it might indicate a problem I need to address.
This is caused by your PAIR valve (aka smog pump, aka AIS) and it's something for emissions standards that dilutes the exhaust with clean air. In a de-restricted (aftermarket) exhaust the unburnt fuel mixes with this clean air and has a secondary combustion event.
If you look up the clean air mod, block off plates, pair valve delete, ais delete you'll get an idea of what that's all about. One of the most popular mods and pretty much mandatory if you're going to hit the dyno or run a wideband o2.
Very easy to do as well.