When is this? There are track works at times. But train is cheapest and quickest - https://www.sj.no/

Otherwise there is the dedicated airport bus (expensive) - https://www.vaernesekspressen.no/

Local bus (#70, #311 and #430) costs the same as train (42 NOK when prebought with app - AtB or AtB Mobillett) departs from just outside the multi-story car park outside of the terminal - https://goo.gl/maps/pHdNw3DdxapatDUR8 (you can walk through the carp park).

Outside of rush hour week days the #70 only goes to Strindheim where you can change to #2 or #10 for the Central Station ((during rush hour the closest stop to the station is Olav Trygvassons Gate) ). #311 goes to the Central Station but not all departure between Stjørdal and Trondheim go via the airport stop. #430 has very few departures.