'charging' sound happening at random

The sound that the Macbook makes when I connect the power cable is intermittently just happening at random, when it's already plugged in. My Macbook is 4 years old now. Does this mean the ports are getting worn out maybe, and connection is happening erratically as a result, even when the cable is plugged in? I have replaced the cable and the charger base a few times because I thought that was the issue. Nothing's touching the machine when it happens so the cable's not being jostled out of place. So far I've not been looking directly at the display when it happens, so I don't know what charging state it's been in before and after the sound.