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So........recently shelled out a lot of "doe" for a brand NEW fluval 406 and 306 because of the "quiet" ratings it receives on almost all reviews (except for forums).Bottom line up front, they are both LOUD! The impellers make a grinding noise. What is most disappointing is that my 406 was silent at first and is now grinding.

I have tried everything:
the teflon tape on the impeller housing
teflon tape under the impeller itself to better lock it into place
purchased all new motor head kits and replaced already BRAND NEW parts
tried silicone lube on all the parts

I have had many canisters and can't seem to figure these out. The filters are properly primed and all parts are in order. The only air in the system is when I break them down to try and fix the grinding noise and restart them. Customer service is non existent.

I have not heard grinding from the Fluvals before. Could the impeller be cracked? How long have you had it? Sometimes its better to go through social media for warranty service (as much as I hate it).

That's strange for a Fluval. Any chance sand is to blame?

I remember ours did this when we first purchased it fresh out of the box and I was about ready to throw in the towel on it too.

Come to find out after looking at all the connections, the red spin valves that control the input/output to the hoses was 'too far' opened. I gave each one a slight spin backwards and I haven't heard a peep from it since. It was the craziest thing I have ever seen/heard of, but that slight overage on the top was the cause in our case and once I brought it back just a hair on each, the noises went away completely.

Give that a check.. can't guarantee it is the cause, but that's what we found on ours...