I recently discovered that heating of certain salts produces crackling sound and that is called Decrepitation. But I couldn’t figure out the reason why that happens moreover I couldn’t find anything satisfactory on the internet.
Frankly speaking, you're reading too much into this phenomenon. Have you ever heard the satisfying crackling noise when you burn logs of wood in a bonfire? I'm sure you must've got the answer by now: Decrepitation is more of a physical process rather than a chemical process.
When we write chemical equations, we treat compounds as single molecules, but in reality this is not so. You specifically mentioned lead nitrate. You must be aware that it is a white/colorless crystalline salt. To expound further, solid lead(II) nitrate exists in a face centered cubic structure. Coming back to your question, when we heat Pb(NO3)2, we don't heat a single molecule, we heat a sample of the salt. When we do so, the crystal structure breaks as the entropy increases. The deformation of the crystalline structure causes sound energy to be evolved.
TL;DR: Breaking down of large crystals into smaller crystals and/or intermediates causes the crackling sound known as decrepitation.