Well, insofar as thought is communicative, it's external insofar as it its communicating to oneself as if one is another; if that's not going on, I wouldn't call it communication and thus not thought, just more experience.

Whether recollection can be purely experiential is a related issue; I think I'd argue that it cannot be because one is almost always recalling something for a reason (and if one is not it's not so much a recollection as merely a sudden intrusion of the past into the present) and thus it is bearing meaning with a system of symbols.