strange smell after faucet runs for a while

Ok I can't figure this out. Whenever I run the faucet in my kitchen for a few minutes suddenly a really bad smell hits the air. It's strongest under the sink. I thought it was the trap but the smell still comes back after I had it replaced. The plumber didn't believe me when I told him about the smell, he said it was impossible.
Anyone have any ideas?
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A leak in the vent pipe will do this

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Clean out the air gap line, esp if a dishwasher is attached. Also check air vents in front of sink if bathroom type.

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