Hey folks. Thought I'd post a couple of general points as I worked for a few years as a system builder for a custom PC company and I've used hundreds of fans of virtually every type and make.
Size - a bigger fan is always better if you have the space to mount it. Fitting a big fan and attaching a speed controller will allow you to slow the fan down to get whatever balance of noise level and airflow you need, while allowing you the flexibility to speed it up in warm weather if required. Running a dirt cheap 120mm fan at reduced voltage will give you more airflow with less noise than the most expensive 80mm fan. It is next to impossible to get decent airlow out of a small fan without lots of noise - a small fan with a high CFM rating is guaranteed to be noisey. Don't bother with anything smaller than 92mm unless you already have the fans to hand or don't care about noise. Most PC fans are designed to run at 12 Volts, but can be slowed down by reducing voltage down to 6V or less without causing the fan to stop. Note that some fan designs cannot be controlled in this way, such as those with built-in temperature-based speed controllers, and different fan designs will stop turning at different voltages. Never run fans at their absolute minimum voltage as tiny changes in environment can cause the fan to stop turning - always run fans at a slightly higher voltage than the point at which they stop dead or start 'twitching'.
Bearing - ball bearing fans last longer and are more reliable than sleeve bearing fans. They also rattle long before they fail, giving you more warning that they need replacing before they stop dead on you. Though more expensive, ball bearing fans are generally well worth their price. If quoted, the MTBF (Mean Time Before Failure) rating on the fan will give you an indication of how long a given fan should last on average (in hours) at full speed.
Mounting - if mounting to a hard thin surface like a box the vibration from the fan will reverberate through the surface it's mounted to. If mounted in the wrong place the mounting surface can act like a drum, turning the vibration into audible sound. Mount fans in corners if possible, or if necessary use dampeners like rubber grommets between the fan and the mounting surface to deaden vibration.
Controllers - there are various fan speed controllers available that will allow you to fine-tune a multi-fan setup easily, and some allow temperature sensors to be attached to control fan speed automatically. If you have a number of fans to control and want to keep noise to an absolute minimum these controllers can be well worth the investment. Avoid fans with built-in non-adjustable temperature control are they are hard-set to speed up the fan at 35-40 degrees C, much too hot for a grow box.
Specialist PC overclocking suppliers are the best place to find good quality fans and mounting/control accessories including ducts and adapters. One I've used in the past for hard-to-find accessories is linked below (I have no affiliation with them). Even if not purchasing from them they have an excellent range of products so the site it great for checking what's available.
http://www.coolerguys.com
I hope this helps.
