Well Handyman, my guess is that 20 years ago when the house was being built someone put a dead body in the slab of your kitchen floor....... and it is just now starting to stink..... I ran into the same problem a few years ago with my old house and I could never figure out what the smell was. I lived with the smell for five years until the State Police and FBI Agents came to my house and told me that a man had confessed to killing three people and that he had put them in the Foundation of my house when it was being constructed. Then it hit me that‚Äôs WHAT THAT SMELL IS.. I asked them many questions which they were happy to answer, one of the questions I asked them is how come I could smell the bodies after all they were in concrete. There answer was as follows. after a NUMBER of years in the concrete the bone starts to wither away and the bone MARROW causes a chemical reaction with the lime, which causes the concrete‚Äôs ‚Äúpours‚Äù to open up when ever there is direct heat over the ‚Äúhot spot‚Äù needles to say I no longer live there. the FBI tore down the house; however they reimbursed me $80,000.00 so I just built a new house.
It could also be one of the following problems....
1. Are you sure your son checked the vents or did he just say he did? We all know how kids can be, I should know because I am one. I was just pulling your leg about the dead body thing lol I could not let that one go. If any one fell for it please leave a message
2. Maybe check the exhaust on the furnace maybe a rat or a bird got in there I do not know how that would make your kitchen stink but who knows it could.
3. if you cant find out what the smell is maybe you should buy a heapa air filter you can get them real cheap On http://www.ebay.com that will help out a little.