I had the same problem with my MB Air 2014 (non removable battery) and read many posts I found in several sites. Tried the SMC reset, and then went to look for spilled liquids in the board. So I followed the ifixit guide to open-up the MB all the way to the front cover. Risky procedure for an unexperienced technician. At the end i didn't find anything dirty or needing to get cleaned. However, I put back everything in place and closed de MB, and turned it on... to my suprise the problem was gone! The MB was running silent and fast! My conclusion, you just need to disconnect the battery for a while. An this is a very simple and safe thing to do. Just unscrew the back cover following the ifixit guide and unplug the battery. Put it back together and Voilá!