Now on to the 2nd part and I see webby has already made a reply
Originally Posted by Webby 
The fan speed contributes to the noise because of 2 things first more air is being moved and you can hear it moving (turbulance) the second thing is the faster the fan is spinning the more noise it makes from its bearings.
Actually there is a third thing, balance and vibration.
Higher quality fans are made with denser less flexable plastics and tighter tollerences on the molding on the fan blades, sometimes they go through 2nd balanceing process where material is shaved off the blades to thurver improve the balance.
The more unbalanced a fan the more it wobbles as it spins and a weak frame alows for more twisting which adds to the wobble and this is transfured into the frame of the fan as vibration.
One simple and extreem experiment you can do here is to break off one blade of a fan and you'll notice that the vibration and sound greatly increases (don't do this on a fan you want to keep as breaking off a blade does effectively destroy it)
Good quality fans have better bearings and so are quieter at higher speeds however you do not eliminate the turbulance noise.
I find that bearings are a bit deceptive, the differnce between a quality and budget bearing isn't as great as you may think, in terms of sound anyway.
Bearing type often plays a greater part in the sound level, but not as much as balance and frame strength does. (also note that unbalanced fan bladed put a greater strain on there bearings thus reduceing there life span)
Higher quality bearings tend to show more in there lifespan than there sound levels, again it's down to better materials and tighter tolerences
There are two main types of bearing, sleeve and ball
A sleeve bearing is 2 tubes one inside the other, often with a single grove running around the middle of each to act as a fluid resurve for the small amount of grease used to stop the two tubes rubbing together. They are more quiet than ball bearings, simpler and cheaper to produce, but have a lower lifespan and when they go the tend to just suddenly sease up.
A Ball bearing is basically the same thing but the groves are deeper and little balls are placed inside them (this is where the name ballbearing comes from) the two tubes rotate on these balls.
They generate slightly more noise than a sleeve bearing and are slightly more expensive to produce, but have a longer lifespan and tend to slowly fail, they slow down and start makeing a grinding noise before totally seaseing up.
What Pob meant about vent noise is that the added turbulence generated from the mesh or grill can make more noise than you may expect.
Yep right on the money, air being sucked through a grill or mesh can create added turbulence and/or vibration in the mesh/grill
One other thing you can try is to get vibration dampers to mount the fans but these often come with the better fans and having better quaility fans will make significantly more difference than any silicon mounts.
Umm a silicon compound is often used as an alternative to rubber in vibration dampers as it has a longer life span and is less effected by heat
I do agree that vibration dampers can help to reduce vibration going into the case especially over screws.
I think what you mean is rubber plugs instead of screws with silicon grommets or washers, which is correct.