New update: I went to Ringwood Performance exhaust today and they spent some time rewelding the back muffler part and tidied up some welds. Cost me $100, drove out and it's still making the noise! They said the noise is coming out of the muffler.

I then drove to Modproject for them to have a look and he reckons it might be my metal cat. I'm so confused!

Basically now I have the option to replace my muffler to an aftermarket one. I had changed the muffler to a stock EK4 one and it was still doing the same thing. I would take this approach if I believe that my DC2R muffler or EK4 can't handle the flow.

Change my cat to a ceramic one, i'm hesitant to do this cause my Magnaflow metal cat is brand new! I paid quite a lot of money for it.

Would there be a possibility this is coming from my Hot Dog resonator which the exhaust shop put in? How would I be able to tell of this is the problem?