You've probably got an air leak on your exhaust that's causing the pops and bangs. If it's anywhere in front of the lambda or even slightly behind the lambda you could be getting a bad mixture into the engine so get it fixed as soon as you can.
Barry
Pops and burbling is OK, but backfires are not a 'good thing' to have.
When I put on my Hoffmann race tube and Hoffmann back box (so, like you, the middle box went) the system was slightly louder and burbled and popped away as soon as I lifted my foot off. Maybe it's my perception, but it doesn't seem to do that very much any more - possibly the ECU has 'learned' and adjusted things around or else I've just got used to it and don't notice.
If one of your exhaust valves is not sealing properly, you will get pops and bangs big time. Remember a friend who had a Peugeot 205 GTI and his was popping and banging big time. When we dismantled the engine, found that exhaust valve on 3rd cylinder was not sealing all the way....