Loud muffler

G heats his shop with a large waste oil furnace.  At least that is what is supposed to happen,.  This winter it was not working properly.  He has spent many hours on it.  And yesterday he finally go it working without billowing smoke all through the shop.  He was thinking of just trashing it!  So we spent most of the day cleaning and organizing in the shop.  The car lift is now cleaned off so he can put the Audi up there and look at the exhaust system.  It is at times very loud.  I was even wearing earplugs when driving.  (Lucky I have not gotten a ticket yet) I took it in for a repair estimate, which was over $1200.  The car is not worth that much.  He had already worked on it when I took it in for the estimate.  The man asked me" Who last worked on this?"  I said it was my husband.  He just laughed and said, "Actually he did a pretty good job."  So hopefully in the next week, it will get fixed.  In the meantime, I continue to drive it.