It is a good thing you can hear the vent outside, means it's venting outside, at least mostly. Is the draft sucking or blowing? Not sure about the us but here in the uk a lot of boiler flues are of a type where there are two concentric pipes, the outer draws air in and the inner vents waste gasses out. So it may just be the inlet air leaking.
If you're not in bed already, turn it off before you go and get the gas man round in the morning. Corosion doesn't sound good wherever its coming from.
― Ed, Tuesday, 3 June 2008 07:41 (fifteen years ago) link