I have a gas Trane furnace, new in 2004, it has started making a loud clunking noise when it starts up and when the cycle ends. What is this noise, and how do I remedy it?
I have a new filter that I can put in.

Thank You.
Sheri Tavens

New filters are always good, but you need to get up close and personal, sitting or standing right in front of the furnace you need to try and determine where the loud clunking noise is? Exactly

Then let us know

Note the timing of the "clunking" relative to the start of the cycle.

Change that filter. You may be hearing popping of ductwork, which is due to a dirty filter among other things.

See if you have a metal frame around your filter and if so see if the filter is secured down, if it is not secured down it is possible that as the air flow starts through the filter the filter raises up and hits the metal of the blower housing causing the clunk and as the air flow stops it falls back to the bottom of the furnace making another clunk

Sheri... It is NOT duct work or filters. I have been experiencing the very same thing and I guarantee it is not these things. Although I am still challenged by the stupid problem, I at least have been told the reason: It is your gas valve sticking upon opening and closing. Then when it pops open, it knocks against the side of the casing or opens with force, so that it makes a 'CLUNK'. While it sounds simple enough... it SHOULD NOT be happening. If you have a new Trane, the installers should be servicing it for you - replacing the valve as it will wear down your solenoid. I am still waiting for mine to be resolved.

Karen from Thamesville, ON

Thank you for sharing. However, there is no way to say for sure that your clunking sound is the same sound that Sheri is experiencing. Sheri did not give enough info to really narrow down the cause of the noise. I have been called in for clunking sounds many times that were caused by popping ductwork(sometimes caused by plugged filter). Anyhow, Im thinking Sheri got the problems solved as this thread was posted almost two years ago. Wish she would have come back with an update, but that's the way she goes sometimes. Thanks again for relating the solution to your noise, and welcome to AMHD.