Eheim noise/ low flow

Hello peoples!

 Got a question about my 4 month old Eheim 2213...
After running silently for a while, it has developed a rattly hum when
running. I cleaned the canister out, squeezed the filter pads, rinsed the
substrate, made sure the tubes were clear, and disassembles the outer pump
components and cleaned them. But it still rattles! And it seems that the
flow rate has decreased. The only addition to the filter media was to add a
thin extra layer of floss right before the white pad. Any ideas what's going
wrong?
Along the same lines...
- When (if ever) should I replace the coarse and fine substrate??
- Do I have to replace the white pad every few months, or can I just give it
a good rinse? Could I use some generic floss instead?

Thanks for any advice,

- Seweryn