Did you find a solution for this? I have just done the RAM upgrade from 16GB to 32GB and now the fan often speeds up to full speed (about 4000rpm according to iStats), often no for reason. Before the upgrade I would barely notice it even if I was doing something intensive.
While I had the fan out I did blow all the dust out so the fan and exhaust are both clear.
I have tried the onboard diagnostics as suggested above but it found no errors.