My girlfriend used to live in Mission Bay by the beach. That's in San Diego, in case you wanted to know. One night I was there, we were watching t.v. We kept hearing knocks on the wall and she thought it was some kids outside. I thought maybe it was a branch from a tree. It kept going on for a while so I decided to check it out. I went outside to see what it was, and there was a tree rubbing against the house. I went back inside to tell her it was just a tree and so we continued to watch t.v. We heard the knocks again, but this time it was on another wall. The only difference was that one side of the wall was in the living room (where we were) and the other side was in the kitchen. No one was there but us so we got kind of freaked out.
I told her it's a ghost just to try to scare her for fun. But the more I talked to her about it, the more I started to scare myself. And it finally hit me that it was probably a ghost. This continued for over 5 months. We got used to it and we weren't scared of it at all anymore.
One day we told it to stop and it did. I kind of regret telling it to stop because then the house just got boring like any other old house. Even though it was kind of scary, it was fun with those knockers around.