Often this is the result of a build up of animal fat from your clothes. If you regularly do a cold wash then not all of the oil from your body comes off from the clothes, resulting in a build up of animal (human) fat.
Try doing a hot wash (at least 90 degrees) on the most comprehensive setting you have on your machine. It may require two goes to get rid of the smell.
This should be done AT LEAST monthly. You can use towels in this cycle as they don't shrink too much and it doesn't really matter if they do.
Cheers,
R :o)
Richard, May 2006