I have a `67 100 ranger with a 352. The engine runs strong and holds good oil pressure, but while I'm in neutral at higher rpm a rapid light knock occurs. I know this appears to be a rod knock. The thing is, when I run the engine at even higher rpm the knock appears to go away. Maybe if I have a bad rod bearing the oil pressure at high rpm holds the bearing together thus the knock is quiet. I have done the wooden dowel trick and the spark plug trick to locate the knocking rod and I didn't do to well because I did not locate the noise. I listened into the engine running at idle with a wooden dowel and I can hear the knock. The engine runs strong and fires right up every time too. Regarding the symtoms what could be the problem?

 

Are you absolutely sure the fuel pump isn't causing this noise?

 

Can it be the fuel pump? What will cause the fuel pump to make a knocking noise? I'll see what I can discover at the fuel pump. THANKS.
Any help will be greatly appreciated!

 

The fuel pump can make a loud "plop plop" noise that sounds like a knock, but I've only heard this at idle. It sounds to me like it may be a wrist pin, which will make a light rapping noise that changes or goes away with accelerator pressure. The noise will be loudest when the motor is "floating" until it gets bad enough to knock under load.

 

There is an arm on the fuel pump that projects into the engine. This arm rides on an eccentric mounted to the end of the camshaft at the timing gear. When the arms pivot point wear out it gets fairly sloppy and will create a knock.

If you haven't ruled out the fuel pump you need to since it isn't a major undertaking.
Pull the fuel pump off and check it by lifting up and down on this arm. If it appears very lose just by lightly moving it then you have the fuel pump noise at least. What have you got to lose if it isn't the fuel pump since you should tear the engine down further to find and fix the problem?