None because what you think are ghosts are areas of energy
The same sounds you make. Theoretically, ghosts were once living beings, so inherently anything that's a ghost is making the same noises they made while alive. That includes interactions with objects to make noises. There are schools of thought in this area that insist that ghosts make noise we can't hear, but the evidence is mostly insubstantial, just like ghosts. Something making noises that sound "inhuman" is likely not a ghost... There are other things that go bump in the night, you know.