Post some engine numbers, type, and code numbers, and we can then try to help you out in your hour of trouble. I suspect the Engine is an Overhead valve, hard to tell without the numbers. If it has the shiney square box/cover, held on with several bolts, it is likely an OHV (Short for overhead valve) On the cover, stamped into the metal will be the words: Model, Type, and Code. These are the numbers we'll need. If not an OHV engine, then the numbers will be either stamped into the flywheel cover, or on a neat metal strip, riveted onto the flywheel shroud, on top. HTH: Rustyj

Thank you! I think that may be it! The burner wasn't on low the two times it happened. It actually was on high. But I seem to recall the Vent-a-hood automatically adjusting the blower to a higher speed right before I heard the most recent pop. Perhaps the increased suction is causing a fluctuation in the flame and signaling the ignitor to fire. I'll pay closer attention the next time this happens.

Possible causes for your problem, Furnace oversized(let's hope not) Blower speed set too high(easy to fix) discharge temp too high(when the installers told you they had not seen that with this "new technology did they have a temperature sensor in their hand) some foriegn matter in the ductwork like a loose piece of metal, tape or insulation(this is most likely your problem as you say you hear it with the fan on On. Here is a link that might be useful: Air Conditioning and Heating Repair Made Easy

It is probably something known water hammer. This happens when a valve is shut off causing an abrupt stop of flowing water. This causes a shock wave to go back down the pipe. When you replaced the walls, you must have changed the mounting distance a bit because of the wall thicknesses. Perhaps now a pipe that wasnÂt in contact with a surface now is. If accessible you could try shimming the pipes off of the surfaces with some foam rubber. In addition water hammer arrestors may be necessary in nearby piping to solve the issue. Water hammer, if often and big enough can be rough on the fittings and plumbing systems, causing premature failure besides being audibly annoying.