Is the dull sound from the outdoor unit of the air conditioner broken?

What you said is that there is noise every once in a while, and when the weather is cold, it is likely that the air conditioner will defrost automatically. It is too cold outside, so it is relatively difficult for air conditioners to absorb heat from the outside, and the frequency of automatic frosting will be higher.

Is the cooling effect of your air conditioner ok? This means that there is no fluorine leakage.

It is normal. Don't worry. It's too cold, and the heating effect will not be very good. This is normal.