Just thought I'd check back in. Here is what happenned. I emailed them on Aug 29 requesting a new impeller. This was their solution which was emailed to me on September 5:
Please try this.
Thank you for contacting us. I am sorry you are having an issue with your unit.
Can you please disconnect your unit from the wall. Make sure that you empty the water in the case.
Please take the motor off the case. Also can you get some Q-Tips.
Now take the motor over to the sink. In the motor you will see some fan blades
please grab it by the blades and pull it out. This is what you call the impeller. Now take
the motor clean it with the Q-tips and water make sure to clean really well where the impeller
rest at. Also do the same to the impeller clean with water. Please grab a Q tip and wet the tip with some vegetable oil and rub it on the shaft. That is the metal piece on the motor. Now put the impeller back in the motor.Please before you put the unit back together, just take the motor to an outlet and plug it in. You should hear the difference. This should take care of the issue you are having with your motor. Please reply.
Obviously this is their "form letter" ...... I mean geez .... why would I need to clean the thing when it was making the noise when it's brand new. Note they said use vegetable oil instead of the standard vaseline fix.
That being said, your guys were right. I did nothing and it gradually got quieter on its own ....... to the point that as of today, it is no longer annoying at all. As a matter of fact, while the "rattle" is ever so slight, had I not heard so many people saying they hear NOTHING except the flow of water, I would have thought the rattle was normal.
Something is really wrong there at Fluval, however in the quality control and manufacturing specification tolerances departments. Looking at the inconsistent reviews on various internet sites, the wildly inconsistent satisfaction with their products for a variety of reasons need to be looked at ....... which I intend to tell them (not that they will care)
So ... thanks everyone who encouraged me to just have some patience. I do like the way these things filter.
BTW: my C3 intake tube only goes halfway down the tank ..... but there is an optional extension for $7 (for a 50 cent part) at Amazon. On the other hand, the extension comes standard on the AquaClear 50 which is essentially the same unit.
Another thing I confirmed after hearing it on a youtube video is that the part number for the motor is the same for the C2, 3 & 4 and aquaclear 20,30, 50 & 70. Thus, if your motor on a C4 goes out, buying a new aquaclear 20 is cheaper than the cost of the replacement motor plus gasket.