Is 'line in' and 'audio in' the same?
Quite probably. Years ago there were standards for audio interconnections, but in the modern world they have all been lost long ago.
Mic in is usually a much higher sensitivity than "line-in" so will invariably be overdriven by too high a level of audio. Most modern recording devices have some sort of Automatic Level Control (ALC) to prevent overdriving the device.
The whine you are hearing is most probably alternator whine which is probably not very noticeable on aircraft headsets, but the recording device in the absense of any intercom audio may ramp up the gain and pick up the whine. My advice would be to connect your line in via a capacitor and small potentiometer, that way you can adjust the audio level for the best results and minimise the whine. A A2 uF capacitor onto the centre pin of an audio jack. Connect a 5 K potentiometer from the other end of the capacitor to ground. Take the audio from the slider (centre pin) of the pot to the recorder with the ground from the barrel of the jack. Adjust the pot for the best recording.