Exhaust making weird noises. [sound vid]

[robbyj]

Ok guys after my extractors and high flow cat went on, all was good, drove it hard a few times naturally to test it out.

Now this weird sorta flapping noise has started when plant the foot.
Kinda sounds sometimes like backfiring.. flapping.. possibly a hole?

or is it just normal now its 'worn in'
The sound doesnt really sound normal to me.

The problem is you have to hear it to understand.. i dont really know how to explain it.

There doesnt seem to be any power loss.. still goes hard.

Listen here anyway (the end of the Vid gives the best sound description.)