I think what made Panaflo fans so popular in the U.S. is its low price rather than its quality. Qualitywise, it's not that great.
No question they became popular because of the low price. But I disagree about their quality. My experience is that they are very good.
I've gone through about 50 Panaflos & I find them very quiet, very consistent in quality, and about as good as most low speed fans at 5V. Almost all fans make some type of chugging noise at lower speeds, noise that is not audible at higher speeds, due partly to masking by airflow noise. It's sort of like firing of sparks in engine cylinders -- at low speeds, the firing of each spark becomes more distinctly audible, while at high speeds, they blur into a smoother vrooom. The low speed noise is often more associated with the circuitry in the fan rather than the bearings, and I don't think the Panaflo is bad at all in this regard.
My PCP&C Silencer samples are pretty good too, but I would not say there is the kind of difference between them and the Panaflos that you describe at 5V. The Panaflo hums more at a lower frequency, but the Silencer buzzes more at a higher frequency. Both, at 5V, are virtually inaudible when inside a case, especially if you decouple mount them in some fashion.
In terms of the plastics, the blades on the Panaflo seem carefully shaped for low noise and smoothly manufactured, while as the blades on the Silencer are thicker, cruder (rough edges), and in fact, smaller.
How many Panaflos have you used? Maybe you had some bum units. Can be caused by shock -- like dropping a fan.