mine is doing it and always has....
Check the line connection and make sure it's tight with no broken or damaged lines. I don't believe there are any pumps or anything in the area, just the charcoal canisters. The purge valve is in the engine compartment I think. Otherwise, if you have no engine or malfunction lights on in the dash, leave it be!
Depending on what state you are in, be careful about messing with it. Some states will crucify you for modifying it