I have a heater in the second floor attic. When I turn it on, I hear the sound that it is trying to come on, but fails to turn on I can hear the sound of the fan sometimes

What kind of heater is this?
The fan may have a bad bearing or a frozen one that's to cold to let the fan spin.
Caution on this It could overheat and cause a fire. Don't use this heater until it gets checked out properly. Many fans if not designed to start in the cold have this problem in the sealed bearings. Again be safe.
If you can out it in the warm and try it warmed up. If it works properly then it's the cold

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