Single Loud Thump

Our problem is this: every week or two we hear a noise that seems to be coming from beneath the house, and my guess it that it is coming from pipes. The noise is a SINGLE loud thump. We usually hear it in the evening or at night (it is loud enough to wake us up). It is rare though, as there will only be one thump every couple weeks. It sounds like a basketball is hitting the floor from the underside. This noise occurs when there are no appliances running and no water has been running for quite a while (say an hour or two). It does not sound much like water hammer, as I have heard that noise in other houses.
Our house is a 1953 ranch with rather simple plumbing (one bath; the washer, dryer, and water heater are in the garage and the noise is not coming from there). There is a crawlspace beneath the house, but not beneath the garage.
I don't know what could be making such a loud, but occasional noise. There are never any other kinds of noises that would provide any clues.
I saw one message posted on an old message board here that had a similar problem, but his noise was coming from a wall.

Warren

I am going to take a wild guess.

You have forced hot air for heat and a metal duct system distributing it through out the house. The duct work is expanding or contracting during the night and at some point,when the conditions are just right, a portion of the the duct pops from that expansion or contraction and that is what you are hearing.

It can be quite loud and some systems can do it more frequently.

Noises can be creepy and come from lots of places. You didn't describe the type of house or what it is made of. It could be the house settling. Especially if it's an old wooden house.. the materials warm and expand all day then when it cools it contracts causing movement in the walls, beams, studs, etc. It's amazing sometimes just how much the materials in a building move..and the movement can cause pops, groans, noises, etc. It could be pipes, ducts, air in water lines, or just hot water pipes cooling and contracting causing a noise where it is against a wall or joist. Locating the noise might take a lot of studying and head scratching.

Maybe a check valve on a sump pump?

I have the noise you are describing from my ductwork, as Cass described. Did this noise begin recently when the weather got colder? Do you hear it more often when your heater goes on more often?

Thanks for your help

Thank you all for your help! My top two theories now are:

1. A result of a heating duct expanding or contracting. It does indeed have a metallic sound to it, sort of hollow, like a duct. It happened only since the weather got cool this year, and it happens in the evening when the heating is running most. I was worried at one time that it was an ignition problem with our natural gas heat, but that is a much much quieter poof sound and in a different part of the house.

2. A raccoon entered the crawl space through a vent that I noticed was torn open not too long back. This morning the dog was sniffing around the entrance to the crawl space and reacting like there was a raccoon there. We have them come around the house regularly as we have a pond.

Thanks
Warren

Sounds like time to get a Hav-A-Hart 'coon trap.... we have armadillo problems here.