What causes the popping or tinkling sounds you hear as hot metal pipes cool off and contract?

Question: What causes the popping or tinkling sounds you hear as hot metal pipes cool off and contract?

Asked by sciencetube145 to Gioia, Iain, Jo, Leo, Mariam on 21 Jun 2010 in Categories: General.
- Mariam Orme answered on 21 Jun 2010:
  
  I don’t really know, but those noises can be a bit creepy when you’re alone in the house!

Question: What causes the popping or tinkling sounds you hear as hot metal pipes cool off and contract?