uzee

I hear a clicking noise after the engine is turned off, I have a 1997 Nissan Altima which I bought a few months ago, the noise seems to be coming from underneath the car. Also, it is giving me really bad gas mileage about 17/gal, I have tried pouring a bottle of fuel injector cleaner but it didn't help. Can someone please let me know if this sounds like a major issue.
Thanks.
Uzee