Haha, I have used this picture on forums for close to ten years and no one has ever mentioned it. This must be where the cool people hang out.
(I saw them a few months ago, amazing)
Anyway... I actually took the hypersphere rings out yesterday but I am determined to make it work so will put them back in later. I didn't even try a mat underneath so maybe I will try that first. I'm sure the springs were correctly aligned so I don't think it's that. I did find some comments about lubing the springs but I'm reluctant to try that at this stage. I'm not sure if the rings are contributing to the problem, it may just be that the sound is only audible when it's not being obscured by the loud noises the NT usually makes.
Thanks for the help so far, I will try a few things and report back...