B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 160; Flight Crew Type 1741
Troubleshooting; Other / Unknown
Aircraft Equipment Problem Less Severe
Flight Crew Returned To Gate; General Release Refused / Aircraft Not Accepted
Aircraft
We finished before takeoff checks and were number 2 for departure heading for ZZZ. As we slowly taxied into position number 2 for takeoff; the trim wheel spun uncommanded from 4.9 to 5.5 and stopped. About 4-5 seconds later the autopilot disconnect aural warning came on along with the AP DISC light; which stayed on and chiming until I clicked the left thumb autopilot disconnect button. We turned on to taxi way and I called Maintenance Control to discuss this. We taxied back so maintenance could troubleshoot. On the way back I did press Autopilot A on and disconnected it. All worked as usual (as far as I could tell); and the Autopilot was not on during the taxi out. Operations did a wonderful job detecting every system in the cat III; autopilot type range and found no issues. They began getting frustrated (I believe); and told us that we must have done something to make it happen. I had to refuse the aircraft and called the duty manager for help downing that aircraft. I asked them to pull the flight recorder to see what exactly happened. I believe I didn't describe the incident well enough to give them the direction to look. The flight recorder was indeed pulled; and copied. I could be completely wrong; and that's ok; but I'm interested in what was the issue. I'm writing this as they did pull the flight recorder in the hopes we can find the answer.
[Report narrative contained no additional information.]
B737 flight crew returned to the gate for maintenance troubleshooting when the elevator trim moved from the preset value of 4.97 to 5.50 without any pilot actuation of the control wheel trim buttons.