Bad gas mileage + lifter noise?

[Ryan32]

I've got an 03 limited, only 50,000 miles on it. I'm starting to get a little bit of lifter noise, and my highway mileage has dropped to under 15/gallon.

Any ideas? The oil is good, the air filter is brand new... I would like to check the o2 sensor, but honestly have no idea how to go about that.

 

[97hb]

I thought I had some lifter noise also, was gonna adjust the valve clearance. Come to find out their is a TSB on noisy injectors. Yeah they are that loud. Could still be your clearance though.