Posted 20 May 2014 - 11:17 PM
I agree that dust may just be causing temperatures to rise and thus causing the fan(s) to kick on.
OTOH, it could also be the fan bearings starting to fail, which then would likely make the fan louder. That happened to me with my original 2006 15" MacBook Pro. The one fan started to fail, which then led to the it making a lot of noise when it spun up due to higher temps. I bought a replacement fan from ifixit.com and replaced it myself. Not "minor surgery", but I believe still easier to do than an iMac from what I understand.
The original poster might want to install something to check the temperature and/or fan speeds. Personally, I use iStat Menus for this, but there are other options I believe.