Well I just tried replacing the Zalman fan with two 80 Vantec Stealth’s, I glued them together and wired them both into the same plug which is controlled by the Zalman speed control dingus. I have them cranked up full bore and the cpu is 1 degree cooler, but the stealth’s have an even worse noise.
Next try is to order 2 more Panaflo 80L’s and try it with that.
I hope you did not mean stacking one on top of another. This will cause much more noise with very little or no extra airflow.
The panaflo are quiet at low voltages, much much better than the Zalmans.