I have a 97 f150 2wd 4.6L with 117,000 miles. I am experiencing a knocking similar to the sound of a diesel. It seems to occur most after the truck has warmed up for a few minutes. It also occurs more when the engine is at low RPM and high load. It does it in in all Park and Neutral in addition to drive. It also does it when the truck is idling in addition to driving, but only after it has been driven for a while. I am trying to figure out if it is coming from the engine or the transmittion. It did trip a P0420 code the other day for Catalyst Efficiency Monitor and Exhaust Systems.

 

Your catalytic converter is stopped up. Got to replace it!

 

is there any way to test to see if this is the problem? What causes the knocking sound in the converter?

 

I've seen Catalitic Converters glowing Red Hot. A dead give away that they are bad. But beyond that I don't know.

Doug

 

when the truck is cooled down take a rubber mallet and tap the cats. if they rattle inside there is your problem.

 

sounds like plugged up and broken cats to me