I ask this because sometimes when I think, while not producing any sounds my vocal cords start to ache after talking/thinking in my head for a while. 
meh
Subvocalization, maybe?
I’ve often wondered the same thing myself. When I’m thinking about something very intently, people will sometimes tell me that I’m talking to myself, even though I’m completely incognizant of doing so.
If you are truly not making any sounds, you are probably tensing the muscles around your vocal cords. You feel that ache in the tired muscles rather than the vocal cords themselves. Some people get tense in their backs, some in their abdomen, you get it in your larynx.
Your vocal cords vibrate when air passes over them - a bit like the reed of a wind instrument, but as others have suggested, if you’re subvocalising, you might still be tensing the muscles that control them
When you are subvocalizing you are still activating muscles in your vocal tract. These can be monitored and translated into text for various purposes.
Si