AC/heatpump compressor problems to be awaited?

[Zwerius]

Hi all,

Today I heard a humming sound with a constant frequency, independant of driving speed. After switching of the heating, the sound was gone.
It was 2°C outside. The first time this autumn that the tempearture dropped this far.
The car has 107.000 km on the "clock".

I suspect the compressor of the heating system (which is the same as from the AC I suppose) is making this sound, based on what I've written above.
Don't think it's a fan, because of the type of sound.

What could it be else if it wasn't this compressor?
Should I go to the dealer to prevent more possible damage? Or can I just "wait and see"?

 

[alohart]

Even a new i3 A/C compressor isn't silent. It's easily heard when outside an i3, so some noise is normal. Also, it's a variable speed compressor, so when it's working hard, it's louder. This complicates diagnosing a failing compressor by sound intensity alone. I don't whether the quality of the sound might change when the compressor's rotor begins rubbing on the compressor housing which seems to be the failure mode that could result in a very expensive repair. Hopefully, others who have experienced a compressor failure will comment.

 

[TedStriker]

Where was the sound coming from? AC compressor is in front of the rear left wheel.