There is a tremendous rush of air into the fuel tank when I remove the filler cap. Whether I'm filling up from an empty tank of half a tank, I break a strong vacuum (sounds like a terrible health problem). Is there some vent I should check or is it normal for the fuel tank to suck in rodents and small pets?

As a side note, I have a nagging rough idle but it accelerates and cruises like a dream.

Thanks,
Sixto
91 300SE