I have a Lennox gas furnace that has started making a noise through the ducting that sounds like rain drops hitting the roof during a good rain. What could this be?

If I had to guess, I would say it is the ductwork expanding as the warm air passes through it. Or the ductwork moving relative to some part of your house, like a floor joist. If you can find where the noise is coming from in this instance, you can wedge something between the duct and the joist to stop the noise.

Good luck finding the noise,
WallyH

Do you have a high efficiency furnace with pvc exhaust and fresh air make up , if you do the exaust is not pitched back enough to the furnace and then to the trap and drain, that will create water running or droplet noise. Mikie D

Do you have a high efficiency furnace with pvc exhaust and fresh air make up , if you do the exaust is not pitched back enough to the furnace and then to the trap and drain, that will create water running or droplet noise. Mikie D

The house is 5 years old so I assume it's high efficiency. The exhaust is typical duct material not pvc . This noise just started I never heard it before.

If it doesent have pvc piping for fresh air and exhaust its not high efficiency but what you could try is look for a ductwork joint and pull out the slips and drive and disconnect supply ductwork and spread it apart and if it stops have a cavas connector installed also your saying you have a chimney or triple wall exaust to the outside or in a chase let me know I'm interested. Mikie D

When you say ducting, do you mean exhaust venting, or furnace ductwork? Where and when is the noise heard.

Check to see if your system is iceing up, could be driping through the furnace, look for ice or frost on the refrigerant lines when it is running. Good luck, Mike