This forum came up in a search for the solution to the grating noise in a Fluval 305. Unfortunately the thread did not seem to correctly identify the problem.

If you get air into your system the Fluval (that means probably most models 205, 305, 405) will make a grinding/grating noise.

Usually this only happens when you clean the filter. However it can also happen at other times.
In my case the filter wasn't flowing well so I gave it a few pumps with the primer... at which time it stopped flowing entirely.
Next day it started making a grinding noise.

Took it apart. Cleaned all the parts and put it back together.

Priming is a bit of an art form:

1) Make sure the filter is full of water.
2) Pour water down the intake hose
3) Try pumping the primer.

If that doesn't work
4) Create some suction on the outlet hose to help pull all the air out of the system

That's usually all it takes: A little suction usually saves a lot of pumping.

The newer Fluvals claim to have an improved pumping mechanism. Presumably they're easier to re-start?