Expert: Brad Sears - 5/18/2009

Question
QUESTION: Hi Brad.  I have a 1967 Pontiac Bonneville with a 400 V8 engine.  After the car is started and reaches temperature, I begin to hear a knocking sound coming front the front of the engine.  I've used a stethescope and it appears to be loudest on the water pump casing.  It's not rhythmic, but sporadic.  It's loudest at idle and seems to get quieter at higher RPMs.  I'm thinkning it's the timing chain stretching and rattling around, or could it be something with the water pump?  Thanks.

ANSWER: The test that I use for timing chain stretch is simple and requires only removing the distributor cap. Once the cap is removed use a wrench to turn the crankshaft until the rotor turns. then mark the crankshaft pulley and the rotor. Now turn the engine in the opposite direction very slowly until the rotor just moves. Now check the marks on the crankshaft and if it has turned so that he mark has moved more than 1/2 inch, the timing chain is loose. To check to see if it is the water pump remove the fan belt and start the engine.
Brad

---------- FOLLOW-UP ----------

QUESTION: Thanks Brad.  I tried both and it definitely doesn't appear to be the timing chain.  I took off the belt for the water pump and the power steering pump and the sound went away.  Then I put the water pump belt back on and it still didn't make the noise.  Overjoyed, I put the power steering belt on fully expecting to hear the noise,  but still the noise was gone!  Now I'm baffled.  I think the next thingI'll try is to take the water pump off and see if anything in there is broken.  It's almost like there is a loose nut rattling around in there.  Unless you have any better suggestions.  If that isn't it, I may just need to drive it until the culprit really breaks.

Answer
I'll bet you have a chirpy belt. Stick your thumb nail into the belt running surface. It should leave a dent in the rubber. If the rubber has become hard they will make noise. Changing the tension when reinstalling the belt may have given the belt a different friction tension and poof the noise is gone. If the noise comes back pour a cup of water on the belt with the engine idling. If the noise goes away with the water poured on it is the belt.
Brad