After water damage fan blows noisily non-stop. Otherwise all ok.

Sounds like you have some liquid damage within the logic board. You'll need to open the system up to inspect things and you may need to remove the logic board completely as the other side will also need to be checked out. Here is the IFIXIT guide you'll need to follow: MacBook Air 13" Early 2014 Logic Board Replacement. If you take the heat sink off you'll need to clean off the old thermal paste and apply a fresh coat so I would hold off doing that part.

Use some distilled water and a cotton swab to carefully clean the logic board and then go over the area again with isopropyl alcohol (85% or better) to help in drying out the water.