Horrible smell from one vent?

During the past week I've been smelling a horrible smell in my bathroom. At first I thought it might be septic tank related from all the rain we've had. But today I realized it was coming from the vent. But it's only from the vent in my bathroom. It smells like rotten eggs. My first thought was that my coils were rotting but I'm sure I would smell it in more than one vent. My heating system is a Goodman and it's 10 years old. My heat kind of sucks and once it gets below 50 it pretty much stays freezing in my house. I live in a single wide trailer that is 20+ years old. I've had it looked at by 2 repair men in the last 2 or 3 years. The first one told me I had rotting coils and should just go ahead and get a whole new unit. I don't have the money for a new unit so I declined. The second repair man, over a year later, said my coils weren't rotting, just dirty. He cleaned them and my heat did get a lot better. Before he cleaned them it didn't work at all. But I digress. Does anyone have any idea what is causing this smell?