I have a strong sewer odor coming from under my kitchen sink.

So far I have done the following:

I have made sure that the all pipes under the sink are clean, tight and not leaking.
I have made sure that the garbage disposal is clean, tight and not leaking.
I have made sure that the dishwasher drain pipe leading into the garbage disposal is clean, tight and not leaking.
I have cut a hole behing the sink/dishwasher and checked the vent pipe for leaks and everything looks dry.

One odd thing is I have a water hose outside near where the kitchen sink is. When I run the water I can smell the sewer smell, when I turn it off the smell goes away.

I am completely out of ideas as what to check. Please help!