I know it sounds weird. I recently started noticing a strange slight knocking sound when coasting in my 99 Ranger (with the 2.5L 4-banger and 5-speed manual). I could actually feel the knock slightly in the steering wheel. After careful listening I realized that the knock is always present but is usually drowned out by my radio and racous engine note. The knock seemes to occur more over rough terrain, as if something was loose. Well, vibrations can travel, so I popped the hood and started wiggling everything. I found that the heater hoses that run from the front of the engine down the left side to the heater box have quite a bit of slack. In the middle of these hoses is a plastic junction which was banging into the AC condenser (a black metal cylinder) leaving a telltale scuff on the condenser. I secured the heater hoses to the intake hose with a Long (1 ft) twisty tie and viola, the knock is gone. Hope this helps someone.