How to reduce low frequency noise in heat ducting?

I had a gas furnace installed to replace the 28 year old gas furnace that’s been providing heat to my house since it was built. The old one was silent in operation. The new one is really noisy at startup. The sounds consist of a vacuum cleaner like sound, followed by a timpani like rumble, followed by the main fan engaging. My chief complaint is the rumble. The company that did the install says the sound is attributed to the burners lighting, they also claim the sound as normal. I had another company check it out and they agree that the sounds are typical of most 80% efficient furnaces.

The furnace is located under the house. The house is a 2 floor structure and you can only hear the noises downstairs. The noise comes through the air vents. I'm faced with throwing this noisy furnace away or finding a way to dampen the startup sounds. Given the cost of the furnace, I’d rather fix the problem, if possible, than start over.

Back to the question at the top of the thread. What can I put in line to reduce the low frequency rumble? I've heard of ducting lining, but given that the ducting in place is only about 8” across, there is scant room for lining inside the current ducting. I'm not against replacing the ducting, if it will work. Using soft fiberglass insulated ducting instead of rigid ducting is a possibility, but someone warned that rodents can easily gain access to the house by chewing through this ducting. Since I can’t hear the startup related noises upstairs, I'm thinking I may get to my goal by doubling length of the ducting to the downstairs rooms.

Any suggestions?