all of a sudden my cooling fan sound has increased a lot. This occurs during winter. I remember it happened last year too; after continuous switching the pc on and off, the sound went away, it started to noise again from the next day.

This year it is occurring again.

what do I do?
Why is this happening?

Thanks

Clean or replace the fan...

the sound persist after cleaning.

Usually that type of noise is due to the bearing getting worn out in the fan motor.

but sometimes when I switch off the pc and on the pc sound goes away. The sound is coming from power supply.

What should I do?

Thanks

Then it is the fan in the power supply...in any case, it is a sign that the fan is having problems.

Sooner or later the noise AND the fan will stop...