I have a 1999 Toyota Corolla LE with about 77,000 miles on it and noticed today a soft knocking sound coming from under the hood. This sound wasn't present when I started up the car yesterday. The sound is like a steady tapping that's only noticeable when the car is in park. I can't hear it at all when I'm driving, and have noticed no handling changes. I'm not sure what this problem could be. I recently got an oil change and have been using the air conditioner on high everyday, I'm not sure if these could be factors. Anybody have any thoughts on what this could be?
Perhaps you could listen carefully with the hood open to try and locate where the knocking sound is coming from. Sometimes it helps if you have a mechanic's stethoscope (about 15 dollars at your local parts supply store) to narrow down the exact location of a noise. You can fabricate a make-shift stethoscope by using a length of rubber hose or vacuum line. Something about the size of the tubing used as air hoses on fish tank aquariums would do. Just insert one end of the hose into your ear (or at least hold it up snuggly to hour ear) and use the other end to probe around any areas that the noise might be coming from. As you get closer to the noise you will be able to hear it better and better through the tubing. Just be careful of any hot or moving parts in the engine compartment while you are probing around.
If you can't tell where the noise is coming from, perhaps you could give us some more details about it. For instance: Can you hear it better standing outside the car or from sitting in the car? Can you hear it better with the hood up? Does it still make the noise if you hold your foot on the brake and place the transmission in drive? Does it get louder if you rev up the engine? Does the frequency of the noise increase with engine speed? Is the noise still present after a long drive or does it change in sound as the engine gets warmer?
Give us any other specifics that might help, and perhaps attempt to better describe the sound. Try to compare it to other noises that most people have heard or would recognize (for instance... it sounds like tapping a pencil on a tin can or it sounds like hitting a board with a hammer etc.) It is very difficult to offer diagnostic suggestions for noises that we can not hear, so the better you describe it the better chance you have of someone recognizing it. You might even try to describe it phonetically. You know , like --tick tick tick tick or ping ping ping ping or tack a tack a tack a tack etc. I promise not to laugh.
These car have all kinds of problems with the serpentine belt tensioner for which reason it has been redesigned. Remove the belt and tell us if the noise goes away. If so then buy the revised tensioner assembly from Toyota.
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