Carry a 2.5 foot length of 3/8" hose with you. When it makes the noise again, put one end of the hose to your ear, and poke the other end around the engine to pinpoint the noise. You can even stick it in the oil fill hole for a listen. Don't forget to test near the fuel pump if you have a mechanical pump. There's a rubber grommet on the pump rod and if it splits the pump will knock.
Another knock I once heard on a pickup truck engine was caused by a very worn out thrust bearing, when we stopped facing downhill the knock appeared, when facing uphill it went away. The crank front counterweight was hitting the front main web on the block.
David
Last edited by David Pozzi; 07-02-2011 at 07:53 AM.
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