... But there are some experts that may reply (so stay tuned.)
For one, it's always hard to describe a 'noise' that comes for an HVAC unit, a car engine, the washing machine, and the like. With that, one can only speculate until an expert is able to see and hear the unit, first hand.
FWIW, I had a similiar noise. MY particular problem was w/the duct, itself. For a moment, there was such a 'draw' on the return duct that it would 'collapse' a bit from the draw when the blower turned on. All I ended up doing was shoring up the duct to a floor joist w/a duct bracket to prevent this from happening. The noise would be similiar to the noise you might hear when the front quarter-panel of your car gets dented, and it gets pulled 'out'. If you push it it again, and pull it out, you keep hearing a kind-of 'popping' noise from the metal. In my case, it wasn't the ignitor or the cage, or whatever. It was the metal of the duct being drawn in.
Your best bet is to have a Pro look at it. Unless you're SURE of the cause AND SURE of the fix, I wouldn't mess w/it. These units are the source of fires and carbon monoxide poisonings! Get it serviced ASAP.