My fx5s will make that grinding sound if I don't clean off the impeller for a long time. It can even stall the filter completely if left too long. When you empty the canister out, u can use a screwdriver to remove the motor housing and then pull the impeller cover straight off, remove the o-ring and the impeller itself (it's a magnet so u gotta yank it a bit). They accumulate gunk over time so give it a good rinse and then rinse out the slime from the inner impeller chamber and put it all back together. Should do the trick.
Sorry if u have already tried this - but if u haven't it is super simple! My fx5 has been running a lot longer than 7 years, so urs may have more life in it yet.