If the noise goes away in gear, it may be the connection between the engine and transaxle. Are not some of those drives in transverse set-ups Hi-Vo chains? I get the same thing on occasion from my wife's 89 Z24. It sounds like chain rattle when there is no load on the drive. Could even be spur gear noise when the load is minimal, if that is how GM got the power between the parallel crankshaft and transaxle.
Current fleet (subject to change w/o notice) \'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run. My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787. Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.
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