Typically that indicates the source is related to something that shifts position when the engine rocks when put in gear. Heat shields on the exhaust system are the best suspects. Most are welded to the pipes, and they often rust apart and rattle. You can find those by banging on the the exhaust pipes with a rubber hammer.
Most exhaust systems today come from the factory welded together, but once parts like the muffler are replaced, those are often attached with clamps. Those clamps can rust too and rattle. Those are not related to putting the transmission in gear though.
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Tuesday, September 5th, 2017 AT 3:46 PM