DD (4 and a half) has recently started making lots of little repeated sounds, particularly when she's engrossed in something like eating/tv/looking out of the car window. It started when she was a little congested, and has continued. The sounds change over the course of a day but she'll repeat the same sound for half an hour. It's never full words but a sound like 'hnk' or 'pia'.
I'm not sure if she's entirely aware of what she's doing but I asked her once and she said she just liked the sound.
Does it sound like a comfort mechanism, or something else?