According to the aircraft.cfg file, the min_gov_rpm is 25520 or prop rpm of 1450.

29920 or prop rpm at 1700, are the full power settings.

I suppose at High Idle, the prop should be at 1450 and the engine at 25520 but that is not holding.  In fact prop rpm drops to around 1048.

You are therefore hearing the prop rpm go through a wider rpm change and therefore the pitch follows (about a 62.2% change in prop rpms).

If you hold the prop rpm at 1450 on the ground and engine at 25520 (about 45-50% power setting), then increase to full power, you will hear only a small pitch change which is more likely correct(about 13.3% increase in rpms).

Not sure why the engines are dropping to the lower value while at high idle position, and the governor is not holding to the minimum rpms unless the 15.2 degree beta is too much drag on the props.  Minimum on ground beta is 1 but AFSD shows props staying at 15.2 beta until I hit the F1 key to set idle.  The rpm increases to 1164 once the on_ground beta is set.

So, you are in fact hearing what the sim is doing, increasing the prop rpms from 1100~ to 1700 (a 62% increase in prop rpms) instead of from 1450 to 1700 which would be less noticeable.

Hope this helps though I am not sure how to correct it.

Yes, I agree.  Not sure why the combustion sounds are there at all.  The prop sounds are nice and the flags are set to vary with pitch.  Seems those combustion sounds should be replaced or removed.