A lot goes on electrically in the transition from track power to battery/BCR power. Since you can mute sounds and still get the pop, I think there's a power-supply glitch in the transition. A battery delivers a slightly different voltage vs. current profile than a BCR which might explain the variance. Combining that with your comment that the engine has "weird BCR issues" suggests there might be a hardware issue on the power-supply board. While you're running DCS and can mask the behavior, a conventional operator could not ignore it with continual pops on direction change and visions of impending board failure.
As to what to do about it, I'd try another known-good battery. I'd swap the power-supply board with another PS2-3V power-supply board. If you have access to an oscilloscope, I'd observe the power supply signals during the transition. In other words, I'd want to get to the bottom of it but that's just my opinion.
