I had a somewhat similar problem. It ended up being worn out bearings in my Idler pulley.
Take a hose and put one side to your ear and the other side to the different places in your engine, try to narrow down where the sound is coming from. I had mechanics tell me that it was the timing chain slapping. But in the end it only cost $18 and the install was simple.
Although the timing chain made sense, it wasn't the problem.
Hope that helps
