Pretty hard to diagnose these type of 'I hear this' issues. Without us hearing the sound, you're pretty much just going to get shots in the dark.

So here's my shot in the dark - are you SURE it's not something rattling in the cabin? Some little squeak, something in the dash that could sound like it's in the engine bay?

Try to have a better listen. Maybe get someone else to drive while you listen for it. Or maybe if you could describe the sound better, we could make some more educated guesses. Sorry man, that's all I got....