Personally, Mojave has caused my 2013 rMBP 15" to go from a near-silent machine most of the time to one that has fans spinning noisily at all times.

Have tried everything short of a complete reformat - NVRAM, SMC reset. Nothing in activity monitor gives me any clues either.

I even rolled back to High Sierra to check my sanity. No crazy fan spin. Reinstalled Mojave --> crazy fan spin.