The front seat occupants don't hear any noise unless all of the above conditions are present: A) The back window is open, B) Speed is greater than 45 MPH, and C) the radio is off or below a normal conversational volume. Even with all of those conditions present, if you listen for the noise, it is a rather low fluttering type noise that is nearly drowned out by the normal wind and road noise. As speed increases to 75 MPH, the fluttering gets faster but does not seem to increase in volume. At that speed, the wind and road noise overpowers the fluttering noise. If here are rear seat passengers and you are traveling for extended periods between 45 and 65 MPH and the window is open, they will hear the noise. It tends to blend in somewhat with the other wind and road noises but it can be heard by back seat passengers. In the 5 months that I have had mine, it has never been a problem. To be fair, I rarely, if ever, have the rear window open when there are rear seat passengers, but not because of the noise. Knowing that there is noise (from a lot of sources), when you have back seat passengers, the obvious answer is to close the window! You can also partially or fully retract the Retrax! I suggest that very few people these days drive with the side windows open when there are passengers in the vehicle. The normal wind and road noise makes it too difficult to carry on a conversation or even hear the radio. Why then is there a concern about only noise from the Retrax that can only be heard with the rear window open? Frankly, I don't have a problem. The noise is insignificant.
I agree with you Ultra-HOG, even though I don't have a Retrax cover. I do have a BasicKoncepts cover that I have never heard any noise out of. But like you, I seldom drive with the windows down. I could even do without the sliding back window, truth be known.
I don't see what all the fuss is about. On a vehicle with such a great Climate Control system, who needs windows? And even on those nice Spring & Fall days, when want want to roll em down, and let the fresh air in, don't you expect a certain amount of wind noise at that time? Aren't you doing this, so that you can experience the feel, smell, and noise of the wind?
Of course, I guess windows could also be lowered to vent the vehicle for smokers. But that's a whole different topic...