This actually happened to me last night/this morning.
I was lying in bed around 1am, surfing through my random thoughts and trying sleep when I hear a vigorous tapping coming from my backpack, which was on the ground next to my bed. It stopped after about 7 seconds. The whole thing was quite random, so I tried to think of some rational, non-ghostly explanation. I was definitely awake at that time, my roommate was in his room, talking with his friends, and I'm pretty sure I don't own a backpack or anything else that "taps" repeatedly. I figured I wouldn't be able to explain it and that trying to figure it out would only keep me up longer, so I rolled over, slapped my backpack just to see what would happen, and then tried to get back to sleep.
At some point during the night after 2am (when my roommate turns off his music and goes to bed) it happens again. I simply waited for it to go away, which it did, and tried to get back to sleep. I lay there, partially awake for some period of time, and then experienced a quick, sharp pain in my back. I have no idea if that's somehow related.
The tapping was definitely real and loud enough to break the silence and wake me up. It sounded like the intensity of the tapping wouldn't hurt someone if they were being tapped like that, but would be more than enough to get their attention.
Thoughts? Explanations? Experiences?