Is it possible to replace the fans they came with, with other fans from another company as long as they are the same size?
Yes, I do it all the time for old units that come in, depending on the positioning of the mount holes in the fan.
Most of the time the fans simply screw in to the top of the heatsink. Get the stats on the original fan that came with the unit; pay attention to the CFM (Cubic Ft per Minute) or amount of air moved rather than the RPMs. I will admit that finding a fan that moves the same amount of air but is quieter will be tricky. As a suggestion, if your main reason for swapping fans is lighting, maybe there are 3rd party lighting add-ons available that can attach to the fan shroud to give you the same effect without swapping out the OEM fan. Just a thought. Try Newegg, Tiger Direct and Amazon for starters.
Another suggestion, if lighting is your main concern, is to leave the original CPU heatsink fan and add LED case fans which will accomplish the same thing.
I will also say that a lot of these 3rd party LED fans are junk; they are noisy and cheaply made. I have several in my stock drawer that I have purchased through the years and occasionally use them in older units (at no charge) when needed.
[EDIT]: Sorry, I didn't fully answer your question about db. Db are tricky; I have seen fans that have lower db, move the same amount of air, but have an irritating whine that is simply bad engineering. That is why I have a drawer full of them. You really won't know until you plug it in. Reading the users' reviews on suppliers' websites will help but it's not gospel. That is another reason to consider using LED case fans instead as I mentioned above.
Edited by ranchhand_, 28 February 2015 - 10:08 AM.