Help! Cat back fitted, now have problem!

[R21FRMk4]

Have had a custom cat back fitted by a reputable company, but now when the car activates Act at cruise speed, I get a deep growl, through the vehicle... Only when eco on dash and act on...

Any advise? Any recommendations?
64 plate Leon fr 1.4tsi
Cheers

 

[Deathjam]

You sure they didn't put an actual cat in there?