HFCs rattling?

[Animal]

Had Berk HFCs fitted a couple of weeks ago (car sounds excellent now), and I've noticed that when I'm coasting I can hear a rattling noise that wasn't there before they were fitted. I'm useless at descriptions, but the rattling sounds like it's coming from very small pieces moving quickly, as opposed to a clattering from larger pieces moving slowly.

 

Is this normal? Do cats need to settle?

 

Thanks in advance, Alex

[P15UL T]

i has a rattle / rasp coming from my sports cats the other week, took it back to the garage and it turned out to be ones of the o2 sensors had came loose

 

it did sound like a loads of nuts and bolts rattling inside the exhaust - very embarrising