There’s a nasty smell when the gas boiler is on

I’ve noticed the past few days that I’ve had the boiler on that there is a nasty smell and it isn’t coming from the room where the boiler is but it’s coming from the kitchen and the stairs and hallway near the kitchen. It doesn’t smell like rotten eggs. It only seems to stink at night time and some days it smells like animal feces and sometimes it smells like cat litter. We don’t own any pets. I called the gas company about a week ago and supposedly he checked for gas leaks and said he didn’t find any but there were three gas leaks here about two years ago so I’m always paranoid. I don’t know why this smell keeps occurring and it was really strong the other day to the point where my child kept coughing. Does anyone know what this could be?  And I forgot to add that the smell goes away when I turn the boiler off. That’s why I suspect that it is the boiler.