I have this happen sometimes. Usually its loud but mundane stuff like doors closing, a familiar voice saying something vaugly important, something slamming or dropping. It can variy wildly but usually its the most boring crap.
Usually I go back to sleep with no issues but sometimes it startles you and gets the adrenaline going. I usually rule it out as half dreaming but sometimes you got to check to name sure no one broke in.
The absolute worse is the cat throwing up, I have to go check that one since sometimes its real.