I have a 96 Isuzu Hombre truck with 91,000 miles.
It has been running surprising and pleasantly great, but today coming
back from a job it suddenly has little power.

Other than that it sounds fine except for a small tapping sound which it
has had for several years. There are no loose cables or tubings, no funny
smells, nothing usual. I could barely get it up to speed on the freeway.
It will still do 70 miles an hour once I go through all the gears.

Any small hill, I have to down shift to make it up.

Any ideas what could have happened...feels like it is not getting gas.
could it be a plugged filter??

Thanks