I Agree. It is some kind of feedback when the IR LED come on. It is a electronic humm kind of similar to picking up an electrical line humm through a speaker. They could put a electronic filter in the circuit somewhere to block this frequency from feeding back through the microphone. It is even possible that this may be the transformer from the doorbell that it is picking up. Although why it only sounds when the camera switches over to night mode is a little puzzling. It may be the night mode hardware/circuit that is releasing the feedback from the transformers. I do not know, but way to many people have this same issue for it not to be a hardware problem.
They obviously will not acknowledge this because they would have to replace a bunch of hardware to those customers affected by the defective devices. It seems like they want to make the customer jump through hoops and then return the item to replace it. The replacement does not fix the problem because the same defect is in the replacements.