When I setup my new 405 it was noisy, and i wasn't happy. I wrestled with it late into the night and I finally got it right and whisper quiet, which is how it is supposed to be, these are the hints i can offer that saved my from a long argument with the LFS..
I pulled the unit down, left the hoses in the tank and uncoupled the impeller cover, wiped it over and reseated it with the loud click that needs to be heard ( make sure you are not putting the impeller cover in place and then helping the 'click' along by squeezing the two tabs designed to remove it, really push hard to seat it in place until they click by themselves, if you 'help it' by pushing the tabs into place, you are in fact making things worse, push it in there REALLY hard to make it click by itself and lock into place.
Make sure the tubes connecting to the aqua stop valve are well seated, and when you do up the locking nuts, make sure they are tight, but not so tight that they distort the end of the tube, otherwise air ingress will result and the thing will never run quietly or smoothly.
It will take AGES to prime the 405 with the little plunger ,and I mean AGES, if that is all you use, you will be there forever, take my advice, whip off the outflow nozzle and suck on the end of the outflow like a demon, when it has a bit of flow on it's own, you are almost there, and THEN you can use the priming plunger.
Once you have it running for a while, rock it back and forth and side to side, you will find s surprising amount of air being expelled from the unit and coming out the outflow, do this every few minutes for about 15 minutes to 1 hour until you are satisfied no more air is actually being expelled, the do it again the next day and there will be a little more air to expel, if you still have air being expelled every day, your hoses are not quite perfectly seated on the aqua stop valve, and you need to reseat them.
The unit is noisy, annoyingly so, for as long a there is air in the system (either captured or induced by s not perfect tube sealing). Once you go through all that, the noise will stop and it will never be a problem again..
I hope this helps, it does not hurt to take off the oring and reseat that, maybe with a little silicone grease/oil or vaseline while you attempt to reseat the impeller cover.. I find when the unit is new, the oring is twisted in the packaging, you need to make sure when you lay it around the unit that it is not trying to roll or twist back to it's packaged shape, this will allow air in, and help make more noise..
Let me know how you go, but if it sounds like a coffee grinder, and the impeller is not damaged, its air in the system starving the unit of flowing water..
