Well thanks everyone for your suggestions as I finally located the problem and it's pretty embarrassing considering how simple and obvious but rekon I at least owe it to post it here.
One of the sides had come loose at the bottom of the box, you couldnt see it easily and the main problem for me I found was that it wasn't easy to locate because I couldn't hear where it was coming from. In the end I worked my way around the box pushing down firmly on the edges until I noticed it stopped creaking. So have put some more screws in and that seemed to fix it.
So as you can probably imagine I am very relieved to finally get my sub back ! Thanks again guys, your help much appreciated.