Quoting Revvy on this one, and he's the man when it comes to this...
"The thing to remember though is that if you are smelling or tasting something you don't like during fermentation not to worry.
During fermentation all manner of stinky stuff is given off (ask lager brewers about rotten egg/sulphur smells, or Apfelwein makers about "rhino farts,") like we often say, fermentation is often ugly AND stinky and PERFECTLY NORMAL.
It's really only down the line, AFTER the beer has been fermented (and often after it has bottle conditioned even,) that you concern yourself with any flavor issues if they are still there.
I think too many new brewers focus to much on this stuff too early in the beer's journey. And they panic unnecessarily.
A lot of the stuff you smell/taste initially more than likely ends up disappearing either during a long primary/primary & secondary combo, Diacetyl rests and even during bottle conditioning."
Just to back up what I said earlier about it being very comoon.