Why does my kitchen cabinet beneath the sink smell?

The cabinet beneath my kitchen sink and disposal smells like rotting food, but I can't pinpoint the source of the smell. Oddly, the disposal itself does NOT smell -- I stick my nose down the drain and it smells OK. But if I open the cabinet doors and kneel down, the smell of rotting food is strong... but I've cleaned everything out of the cabinet. I have put ice cubes, lots of citrus peel and baking soda down the disposal to clear any smell, but again, the source appears beneath the sink and not the disposal from the top side. (House is 10 years old; disposal is 3 years old.) There are no visible leaks beneath the cabinet. Two hints:1) While the disposal from the top doesn't smell; the pipe beneath the sink does; and 2) the smell is strongest when I run the dishwasher, but it comes and goes all the time. Any thoughts on what could be causing the smell or ideas to fix?