I own 8 year old Lennox ML193UH natural gas furnace. About once every 4-5 days when the burner fires up it will roar, similar to a jet taking off, for about 3-4 seconds. After the roaring start it will work fine for the next several days and perhaps 60+ ignition sequences. There are no error codes being set. The 'roar' is about twice as loud as the normal noise of the furnace normal firing noise but as I said only lasts a few seconds. Any suggestions as to what is going on. Thank you

Hi. The power switch, on/off power switch mounted to the exterior side of my furnace (not the breaker), is badly rusted. It's inside the house, in the utility room in my basement. Natural gas, 93% or there abouts, efficiency unit. It vents out the side of the house via PVC. The only other appliance is that area is a natural gas water heater vented thru the roof. There's no visible water or condensation inside or on the exterior of the furnace. The room is dry. There is a condensate pump that drains from the furnace to the home sewer waste pipe ( PVC from a bathroom on the floor above) via a flexible plastic tube which is not blocked. The pump drains. All were professionally installed several years ago and have worked flawlessly. Back to the switch - the AC wiring is fed from the breaker box, located in another area of the basement. At the furnace the wiring comes down from the ceiling above (open basement ceiling, all work exposed) via a PVC tube to a metal, new-work electrical box housing the switch. The box is screwed directly to the furnace side. The switch is on/off just like your basic light switch but heavier duty rating. The box interior, the metal cover for the box and the switch itself all have some fairly severe rusting. But it functions fine. Heat and AC all work. The switch works. But why is it rusting?? It's been getting worse over time, as you might guess. No water/moisture apparent in the wiring or tubing. I want to get this figured out. I plan to finish reno'ing the place and hopefully sell end of summer or early fall this year. Thank you in advance!