Sweater is made of wool and generally shirt we wear is a combination of cotton with some synthetic fibers. The rubbing of sweater and the shirt causes a friction between the shirt and sweeter. And, it lets transfer of electrons from one material to the other. This results in building up of electric potential. When enough potential has been accumulated, while taking of the sweater it discharges and transfer of electrons take place. Passage of electrons through air in the forms of spark let out heat and sound energy. Thus, we hear the crackling sound and if we are in a dark room we may also see the spark lights as well.