If you hear the sound under the Hood. Open the hood, and look for dark traces of 'soot' on any part of the exhaust system from where it bolts to the engine down. If there is a leak, the soot will have come out of the leak.
OR
have the hood open and have someone listen for the sound as you sit in Neutral and rev the engine a bit. You did say you can hear it briefly when the car is in Neutral. hopefully a partner can help narrow down the location of the noise.
Does it seem to be noisier when the engine is cold?
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.lyse.net/brox/TonyPage4.html http://cleanflametrap.com/tony/
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