If we take a look in the triboelectric series, we find that wool becomes more electronegative when rubbed against human hair. When we wear woolen clothes, the initial charge on the wool and body is zero. When we take out the clothes, the wool gets rubbed against our skin and get charged by friction. During this process, Q amount of negative charge is gets transferred to the wool leaving our skin with a +Q charge. It is this transfer of charges that cause the crackling sound.
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