My chevy, 250 (not stock), has a noise, kind of a knock at start up, until it's warm. Then it goes away. I've use my stethoscope at the top end and bottom end, all around and can't find the source. It runs and sounds great after about 3 to 5 minutes. A good friend, mechanic / restorer of older exotic cars, agrees that it sounds fine - he is unfortunately about 6 minutes away so he never hears the noise.
Any ideas? Maybe an exhaust leak that swells and closes after temperature rises?
