Air Carrier Captain reported a fumes event during boarding. Passengers were deplaned while maintenance preformed a Technical Service Manual procedure which eliminated the fumes. The flight departed safely thereafter.

Narrative

After removing external air conditioning and selecting APU bleed ON; the FO (First Officer) and I smelled a mild musty odor from the cockpit air vents. I selected Pack 1 OFF; no help. I selected the APU bleed OFF and the odor stopped. We were mostly boarded at this point. I asked the FAs (Flight Attendants) if they smelled anything; and one of the aft FAs said she smelled the musty/dirty socks odor too. I instructed the passengers to deplane and had the FO call Operations to let them know what was going on and to send Maintenance. I advised Dispatch via ACARS. I wrote it up in the logbook; filled out the air quality event reporting sheet; and told the Mechanics what happened. The Mechanics said they would do a pack burnoff. I made sure everyone was off the airplane and made sure the FO and FAs were OK. No one reported any symptoms or required medical attention. Two FAs told me they noticed unusual smells upon entering the aircraft. One aft FA said she smelled a moderate vomit/musty smell. The chaser said she smelled a mild vomit/musty smell. I called Maintenance; our Dispatcher; and the Dispatch.Maintenance returned the aircraft to service and the crew got back on board. The logbook said troubleshot per Manual; Operations check good. The APU bleed and Packs were on. I walked up and down the cabin and spoke to the entire crew. No one smelled anything anymore. I called Operations to request Maintenance come back to let me know what they did; to help me decide whether to accept aircraft. One of the previous Mechanics came back with Maintenance Supervisor; who had not been present before. Before I had a chance to say anything; Maintenance Supervisor stated Maintenance had followed the [Service Manual]; and he would not provide any information whatsoever about what they did. One of the Mechanics who had performed the burn off started to speak; but [Supervisor] stopped him. [Supervisor] said I could call Maintenance Control; who could call the Mechanics; and Maintenance Control could relay the information. I explained that I wanted to know what Maintenance did for safety reasons; to help me decide whether to accept the aircraft and to know when there could be a risk of the fumes returning. Maintenance Supervisor [name] again stated he would not provide any information.I called Maintenance Control; who called the Mechanics. Maintenance told me they did a pack burnoff using the APU bleed; the engines were not started; and no more odors were noted. I spoke with the entire crew; and we decided to accept the aircraft. I briefed the FAs to let me know immediately if the musty smell returned; particularly after engine start (this is part of why I wanted a debriefing from Maintenance; now I knew the engine bleeds had not been tested).We boarded; closed up; then waited over 30 minutes for a push crew. No odors re-occurred. I called the Chief Pilot on duty after arrival.