I have the same problem and it's because the pipe is long before a silencer and it's the noise resonating in the pipe, it's sound crap I know. I'm gunna try a small silencer in the de cat pipe and I'll let you know how it goes
Check you have no rain water leaks mine was bad for steaming and was leaking as sunroof drain pipes blocked and rear lamps raining in, sorted them and now it's sorted, I'd only be concerned about the matrix if your coolant is going down
Managed to get the grinder in and cut the nut in the middle then hammer and chisel I'm afraid was a ball ache , drop the O2 sensor with the cat easier to remove off the car and refit before installation then just plug in. Hope this helps