1994 Disco 1, 171,500 miles. After engine has warmed up and truck has been driven for a few minutes, it is exhibiting a "knocking" type of backfiring at idle. Kind of a rat-a-tat-a-tat-tat sound. With the truck parked, Putting my hand near the tailpipe opening I can feel the pulses when it does this. This is when the engine is at idle, it clears up when I accellerate. Any advice?

 

Do you have a check engine light on? It could be electrical like dist, plugs or wires or it can be a burned set of exhaust valves. May want to do a compression test.

 

No engine light on. Will inspect plugs, etc.