The motor groans

While nearly at work this morning the hub motor on the Gelato started to make an odd groaning/grunting noise at low speed. At higher speeds the sound stopped. The sound made it seem like it was working hard to get started. Luckily it was less than 100 yards from work.. I put the bike on its stand so the back wheel was not on the ground and turned the throttle. It still made the same noise. At one point the wheel did not even turn without some urging. Once it was going it turned without a problem. All of this was after about 4 miles of riding.

An additional datapoint is that the bike does not seem to be coasting as far as it did a week or two.

What could be the problem? What should be investigated first.