1997 Honda Civic a loss in gas miles...

I have al ways been good to do the car maintenance stuff to my car. I'm finding a big problem with my gas mileage. I have went from 41 mpg to 32 mpg and can not find the problem. I have done a major tune up and still no increase in milage. The only i have not changed are the O2 sensors and the injectors and the fuel pump. I have noticed a noise when driving my car. The noise is heard only in 2nd and 3rd gear and at about 2500 rpm and the foot has to be on the gas petal to maintain that rpm. once you let off of the gas pedal the noice stops. i have noticed that my milage started to decrease once this noise was present. If you have any thoughts or ideas of what the problems is or how to get my mileage back up please let me know.
thanks.