There seems to be a vibration that sounds to me like an unbalanced clothes washer or a very loud clock. I can hear it when the furnace is not running. The system is located in a crawl space under the house and I am not knowledgeable or mechanically oriented but I would like to have an idea what it might be before the HVAC people come so that I can give them a really good description of my problem. It is more the vibrations rather than a loud sound.

If you can hear it when the furnace is not running it is probably not the furnace.

Yep, furnaces tend to be very quiet when not running, had a aunt one time that talked to her kitchen stove,but never heard of it making a sound back to her