(I'm) (just) thinking out loud.
Fig. I'm saying things that might better remain as private thoughts. (A way of characterizing or introducing one's opinions or thoughts. Also past tense.) Sue: What are you saying, anyway? Sounds like you're scolding someone. Bob: Oh, sorry. I was just thinking out loud. Bob: Now, this goes over here. Bill: You want me to move that? Bob: Oh, no. Just thinking out loud.