A fan will also add noise, no matter how slow a fan you try and choose. You can easily add a 12V fan and just use an off the shelf 12V wall wart supply, as long as it has enough current for the fan you choose. Places to get both are partsexpress.com, digikey.com, mouser.com, etc. You would want to get a slow/low CFM spec model as they will be the quietest.

Easy way to measure is if you have a digital thermometor like the ones that you use in your mouth, you can place near the vents to get a close enough estimate.