I have a curious problem with an ECM motor on a gas furnace. On start this motor will rock as much as a quarter turn in each diredtion, maybe even stop for a moment, and the finally take off. This whole process may take about ten seconds at the most. All the time this is happening, the motor acts like it is bucking or fighting itself and is making a low frequency rumble.
The motor acts the same way even with the ECM tester from the OEM.
Has anyone seen this before? What is the fix?