My 97 4runner makes a sound that resembles someone tapping a tambourine with the heel of their hand. Definitely comes from under the hood. Makes the sound randomly, not at a particular RPM nor a particular road speed, and NOT all the time. Particularly not when I take it to a mechanic or take them on a test drive. More often the sound occurs between 45-55 mph, but has been heard at less than 10 and over 75 mph.