I bought two Lennox Elite series furnaces for my home from a different company five years ago. Every time that the furnaces started up they made a terrible humming noise similar to a ship's fog horn. The units were under warranty and over the years the installer replaced numerous parts but nothing fixed the problem.
I contacted Fred from Cozy World Inc. who is a premium Lennox dealer and he said that although this is not a common problem he told me that he would contact the Lennox head office in Dallas and personally find out what the problem was and how it it could be fixed.
Sure enough he found out that this happens in some furnaces that require a long line of three inch venting and the problem can be fixed with a burner that was designed differently. Fred ordered the two new burners that were under warranty and after he installed them the problem was solved. He only charged me $200 for his labour which I was thrilled to pay. I just wish that I would have used Cozy World from the very beginning. It is a great company with great customer service.