How do you stop the crackling sound in a Baseboard heating system? How much would it cost to get it fixed?

There are usually plastic slides that clip on the bottom of the element where it rests on the enclosure brackets. This allows the element to slide as it expands and contracts without making noise. Sometimes they're missing or not properly placed and the fins sc. Sometimes the aluminum fins crinkle as they heat up. It's also possible that the copper pipe rubs against the wall or floor as it moves during heat up.