We have just bought a home with a Lennox Pulse furnace, High-efficiency, installed in 1986. (Model # G14Q3-80-C3, if that helps anyone.) This may go without saying (i.e. this is our first home and I don't know what's obvious and what isn't), but it uses natural gas and is a forced air furnace.
What a noise it makes!
When it first switches on, there is a click and a low rumble, but then there is another click after about ten seconds and his immense racket comes out of it for about a minute. Then the racket stops and the air continues to blow. When you take the front panel off, there is the main box (about 8-3/4" x 17"), then a 7" tall, 3" diameter cylinder just to the right, and the racket seems to be centered on that cylinder.
I have tried touching all visible nuts to see if anything is loose, but they are fine. It has been a few months now, and the sound is unchanged, so I don't think that anything is coming progressively loose. We received one opinion stating that it's normal! I hope not! Can anyone comment?