The noise you hear could possibly be gas pressure. To check that simply wait for the noise to appear and then rotate slowly the gas kok shutoff valve on the gas line. Just turn it slowly and not all the way closed, if it's gas pressure the noise will slowly go away. Another noise it could be is iff the furnace filter is dirty, and its a down flow furnace, the whislte may be from the furnace starving for air. Simply remove the filter. I doubt its a cracked heat exchanger, more likely a bad inducer motor prblem or something like gas pressure.