B737 MAX 8 Captain reported hearing a rumbling noise from the aft end of the aircraft during climbout. Maintenance determined that the APU door had remained in the full open position on the ground and in flight without a door light caution occurring.

Narrative

As pilot flying on climbout phase of flight; I noticed a consistent and ever-increasing rumbling that seemed to be originating from the aft end of the aircraft. Forecasted chop was anticipated and reported real time. I was skeptical that it was the weather causing it. I also did a fuel calculation from dispatched and anticipated fuel flow in comparison to actual fuel flow; and the aircraft was burning more than planned. We still landed with plenty of fuel for FAR legalities and Dispatch. I briefed the oncoming Captain in ZZZ1 of the possible issue. He took it on board; and he too experienced the same issues enroute. Subsequent Maintenance investigation showed the aircraft's APU door remained full open in the ground position; which is 45 degrees; while both on the ground and airborne. Inflight is supposed to be 17 degrees open. Moreover; there was no door light caution; which illuminates if the APU door is not in the commanded position 2 minutes 45 seconds after shutdown.It should be noted that Maintenance says that this happens a handful of times with the MAX 8 without the illumination of the door light. The actuator could be at fault. It is a performance limiting MEL.Suggestions: Maintenance investigation into the MAX 8 APU door actuator and door light caution indicator; and why there is a disagree.