We just bought this house. There are two heat sources, one for each floor. The unit in the attic is making a funny noise when it fires up. It sounds like coffee percolating or something whistling, along with the normal burn sound. I've never had forced air heat and, being in North Carolina, I don't know if this is a heat pump or a furnace. I do know it is a combo HVAC unit. Anyway, when the kid from the local gas company was installing the propane stove, I had him listen and he said it sounded like the furnace was "lighting into the flue". What is this, is it serious or dangerous and what do I do about it?

Never heard of such a thing. Can't tell you about the noise without hearing it myself. I have on many occasions seen an LP furnace ignite inside the burner tube, it makes an odd whistle sound. It is dangerous and very inefficient if it does this, causes are 1) Wrong LP output pressure to manifold 2) Bad burners 3) Incorrect primary air adjustments.

"lighting inside the burner tube", well, that might be what the kid said. We have a local guy coming out this morning to take a look at it. From what I described to him yesterday afternoon, he said "well, most people just replace the whole unit" which didn't make the wife very happy, she wants to sue the seller. As they've moved out of state, I don't think we'd get very far with that. In NC, they have a clause in their real estate contracts that says if the unit is "working for the purpose intended" it's good to go as far as inspection goes. The thing provides heat but obviously it's not working for the purpose intended if it's doing so in a manner that's dangerous. But that's a matter for our attorney and not relevant to this forum. The neighbor had to have her combo unit replaced and she said it cost her about $1500, which is a whole lot better than what we had to pay in NJ ($6000) for a new furnace. Thanks for your reply and we'll see what the guy says this morning.