Loud engine and sulfur smell 1996 Chevy Lumina Van?

I took my van to Seattle last year and it went along fine, on the way home we occasionally smelled a sulfur smell, it was not very noticeable and we passed it off as being something in the unfamiliar areas we were in. lately it did it again and now when accelerating the engine is loud. If I remove my foot or let it idle, it is quiet but if accelerate or maintain speeds it is really loud..I'm afraid to go above 50 with it..any idea of what it could be?