Humming Noise in Engine

I have had a reoccurring noise that only seems to happen once it gets Hot outside, sometimes it happens after I turn on my A/C and other times I may not have run it for a few days while driving and it still happens. I will be sitting at a stoplight and my car starts making a loud humming sound, as long as I am depressing the gas pedal it goes away but as soon as I take my foot off the gas it starts humming again. I took it in twice and the dealer drove it for a day and it never occurred while they drove it (of course), but it very annoying and quite load. Any ideas what may be causing it?

1999 Ford Explore Sport (2 Door) V6

possibly something is resonating or producing a sympathetic vibration in response to the engine.

The V6 in your truck, like most V6 engines is inherently slightly unbalanced, due to the V arrangement of the cylinders. If it was in-line or horizontally -opposed (flat six, like a Porsche) it would have a perfect primary balance.
Anyways, the imbalance no longer is noticable when you rev the engine a bit, but often is worst at idle.
Now, there may be something that is slightly loose attached to the engine that is resonating due to the idle vibration. Often heat shields around exhaust system components do this. Sometimes you can tighten up the appropriate mounting bolts or secure the component with a hose clamp (if the mounts are rusty or its rivited/welded on).
But unless you are sure, have a mechanic inspect all heat shields and engine components for looseness.