It's pretty much universal, but after time and experience becomes indistinguishable from "free thought". There are multiple theories on this particular topic, but my favorite is the concept of "Cognitive Language Dialogue". Briefly summarized: you are having a conversation with yourself that defines a subjective momentary reality. If I were to say to you, RED STOP SIGN", you would momentarily visualize one based on the meta-file of similar images in your memory data base. With time and experience, most people are able to mute the "voice" you hear in your head, but it never completely disappears...although it may flicker for only a micro-millisecond. The phenomenon becomes much more complex with speakers of multiple languages, who not only have to choose the right concept images, but sort them from different cognitive sources.My advice is: just keep practicing!