Poltergeist?

Even though our house is relatively new I think it has a poltergeist.  When we first moved in we kept hearing strange sounds from upstairs, and because there are only 2 of us we knew no one was up there.  We'd go and look and of course there was no one there.  Then after awhile when we'd hear a crash from upstairs we'd go and find a shelf down or some of my antique cups on the ground, it got really spooky.
Now a poltergeist, from the German poltern for knock or rumble and geist for spirit, is thought to be a manifestation of a mischievous or sometimes malevolent spirit sometimes connected with a child or adolescent.  Well there are no children living here but there use to be 3 teenagers, 2 of whom had there rooms upstairs.
After a while the sounds and the damage stopped .  Then last week we began finding the door to the attic open and neither of us had been near it, and then a couple of days ago a shelf came down in the pub and stuff was thrown on the floor, luckily it was pewter mugs and not china ones.  I'm getting nervous.