What can cause strange rattling noises in my house?

We have a propane gas heater in the living room, a electric space heater in our bedroom, a television,computer, V C R and D.V.D. in the living room.This troublesome noise sounds like a rapid fire telegraph signal and happens at any time of the day or night. it,s impossible to locate where it comes from and equally loud anywhere in the house Save me the time reading the answer and yourself the effort if you think we have a ghost living with us