I had an exhaust leak from a broken bolt in the block of my 240z, to the headers. It caused the same sound. If you can get your hand near the noise, fell for air movement, if you can narrow it down to a loose bolt tighten it up, otherwise like he said above me you may need to replace your intake/ exhaust manifold gasket. Its like 20 bucks and 2 hours worth of work, but if you don't get it taken care of, you'll definetly hear with your exhaust and you loose horsepower.