Foghorn noise when hot

I have a 1994 E350 on my 18' RV. It has 70K miles, and runs fine except when the weather and engine are hot, the engine emits a strange foghorn-like sound at low RPMs. Once I press on the gas, the sound goes away. I live near the coast so it never happens when I take it to my mechanic. It only happens when I'm out in the desert, and the RV is under a load. My mechanic said it's probably harmless, but I'm not so sure. I suspect something like some smog control device, but I really don't have a clue. Does anyone have any ideas as to what might be making this sound? Thanks in advance.