Exhaust burbling and pops

[Sike999]

Hi there. I own a 2005 Ford Fiesta mk6 standard zetec with 50000 miles on the clock 

the problem is the old exhaust was rusted so I had the back box replaced genuine Ford part after a few weeks driving is was very loud burbling when changing gears or on deceleration and idling deep exhaust burble took it back to garage replaced it and I also asked to replace the centre pipe as well still genuine parts. Again was fine for a few days but now it’s still making the same noise it seems when I park up and leave it idling the exhaust is quiet but after a minute the deep burbling sound comes back 

it passed its mot 2 weeks ago with no advisory’s emissions where fine 

hope someone can help or had the same noise I will try and get a video of the noise