Noise is very relative e.g. an little 0.049 engine screaming at 9 feet at 20K will register far lower on an Sound Pressure Level meter then a Quadra turning at 8K at the same distance. But I have yet to meet anyone who would not rate the screaming 0.049 as the noisier of the two.

Ditto for helicopters - A heli with a louder muffler hovering 9 feet away and 10 feet up that has a rotor speed of only 1400 may not sound as "noisy" as the same heli, with a quieter muffler under the same conditions with the rotor speed bumped up to 1900 rpm.

That being said, at the same rotor speed, the quieter muffler would of course be considered less noisy if all comparisons where made at the same rotor speed.

Considering the above, using a rotor speed of 1600 you will find the following all to be very quiet (under 84db at 9 feet in a 4 foot hover) and delivering good power - the K&S units, some of the Hatori units and the inexpensive Century CN3033 (30) or CN3033B (50) - these are all much quieter and provide better power then the stock ones provided by Hirobo, TT, JR or Century