Hi, I had the same problem with my GPS900 tank. The bio balls do seem to rattle and the is also a tendancy for the pump to vibrate in it's mounting. You can try to loosen off the pump pipe to the filter and carfully align to get rid of the vibe. I also removed and threw away the bio balls and standard filter material. I replaced the bio balls with those clay/ceramic beads (they work a lot better and don't rattle) and the filter with the white filter 'wool' and black foam you get in shops. ( did keep the activated carbon but will change them every 6 weeks. Hope this helps!
