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My guess is that you aren't closing up and little nasties are getting through.
I am also thinking it is something like that.
I am usually having the light flicking and some times some sounds like heavy breathing, laughter or steps.
I am only studying; by the way, I am not practising (except of some experimentations, which are too sporadic and disorganised to be called practice).
I never considered it to be anything dangerous, but maybe I am wrong...
In general, I am simply ignoring all that stuff and starting to pay attention only in rare occasions that these things start getting intense, in which case I am simply recording the occurrence. I never have taken any measures to prevent these things from happening, since I also haven’t felt any malice. I don't know if this approach could be completely wrong for people who practice.