Very strange smell. Stumped

[Grumpyjoewv]

Well, I replaced the boot and it definitely helped, but I’m still getting the smell. It only happens at night when it starts getting chilly outside and I’ve noticed now it’s worse while I’m sitting still. With the engine off, it gets worse and worse for about an hour and then it’s gone. I’m thinking it’s gotta be a gasket expanding and contracting.