Weird sound just started...

I did a search and found some similar articles but no solution/explanation of this weird noise I'm getting all of a sudden... Every once in a while, I hear this really odd sound coming from my engine, the only way I could explain it is like a low droning or humming sound. It sounds fairly loud inside the car, I'm not sure about outside the car. The first time it happened I had started my stang cold and let it idle for a few seconds parked, all of a sudden the sound started and slowly got louder. When I pressed the gas it went away. Today, when I was driving it started to do it, and again, I pressed the gas a little more and it stopped.

The only thing I've done to my car recently was I flushed my cooling system and added one of those "stop leak" products because I had a leak in my radiator. I've since replaced my radiator and put in new coolant but I didn't do a full flush...

Any ideas??

The problem started after I did the flush and added the leak-stop chemical - before I swapped the radiator. The problem is that it happens so sporadically, the only thing that seems to be consistent is that it seems to happen shortly after the car is started from cold.

The sound is so weird, it's not a sound I could ever imagine a car making. It's like the sound you hear when you blow over the top of a large bottle, but louder and deeper. I agree though that it is probably something to do with the cooling system, I wonder if I should do another full flush and refill to make sure that chemical is washed out?