My 94 Ranger pickup has been slowly developing an annoying symptom recently. It's starting to make what I would describe as a lugging sound, even at highway speeds, regardless of gear. I first noticed this while pulling a moderate hill in 5th gear at about 65 mph. The symptom came and went, but now it seems to do it all the time, even on flat terrain and even in lower gears. It doesn't do it through town, it only seems to occur at speeds above 50 mph. I'm hoping someone here might know what I'm dealing with.
Some specifics of my vehicle: 6 cylinder, 4.0 liter, 5 speed manual transmission, 131,000 miles. New clutch about 20,000 miles ago, new fuel filter about 5,000 miles ago, new plugs about 15,000 ago.