Lennox G26Q3-75-1 thumping noise before induction motor starts

[GRS12]

Hi All:

I have a Lennox G26Q3-75-1 furnace. Just before the inducton motor starts, there is a loud noise (thumb) that seems to come from inside the walls.

At first I thought it was the house moving (getting cold out, figured the house will groan a bit), however this has been a constant thing right before the furnace kicks on.

I've sat next to the furnace listening for the thump..no luck. When upstairs I can hear the thumb quite clearly. And again, only happens as the furnace is about to cycle.

Any ideas?

Thanks,

Glenn

[applianceman18007260692]

Have you checked the return air vent pipe to see if it is loose?

[RegUS_PatOff]

thermal expansion of the vents from cool air entering ?

[applianceman18007260692]

That is what I am thinking. Is their a rain flap mounted on your inducer vent?

Edited December 6, 2011 by applianceman18007260692

[GRS12]

Gentlemen: Thank you for responding. I've checked all of the pipes/ducts that I can reach, everything seems tight and secure. There is a rain flap, but when the flap moves due to wind or whatever, the sound is quite different.

Thermal expansion/contraction does play a part of this, however, I don't believe all of the thumping is from expansion. Some of the thumps come right before the inducer motor comes on. As I said, I've sat next to the furnace thinking it may be a switch or something, and there is no unseemly noise there when it starts up.

Very odd.

Again, thanks for your replies.