From looking at stationary heat pumps, even the ones that don't heat directly with resistive elements still have resistive elements in order to 'defrost' heat exchangers. I have to wonder how they control how that defrost cycle is activated - and whether battery cooling / heating is liquid-direct or has an air intermediary.
Interesting that the fan gets noisier when resistance heating goes on. Is it switching from outside air to recirculating cabin air? It may not indicate that it's doing that, but do you get a similar noise change when you switch to recirculating?