My dad's '83SD has been making a "clonk" sound when you start it up recently, it used to be a very small sound, but now it sounds and feels like the tranny is slamming against the sidewall of the tunnel when it starts up, the sound is a muffled "poom!" emitting right from the driver side of the tranny tunnel and it only happens when starting the car, which always works fine. Is this a symptom of a worn out tranny mount? Or something of the like....his motor mounts are practically brand new, the engine sits in there like a rock compared to mine, which roams the engine compartment at will..... So I was just wondering what this could be, its starting to get annoying.....

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See if you can get a 'helper' to start the car while the hood is up and you watch and listen.

See how much movement, which direction, etc. Put a hand on the body near the engine, see if you can feel a thump.

Just some ideas.

Best Regards,
Jim

Yeah, a stethascope can come in handy at times like this.

Danny

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Sounds like a candidate might be a worn flex disc, you may have to get under the car and do the push, pry, pull, and prod routine to check them.
I would no recommend getting under a car when it is started. The flex discs are hard to get to. They are buried up there under a pan on that model. May have to remove the pan to access them.
They should be firm, no looseness at all. Replace them if there is.

CHECK THE EXHAUST HANGER . IT WILL MAKE THAT EXACT SOUND IF YOUr EXHAUST SYSTEM IS LOOSE AT ALL AND BANGING AROUND.
john m

The above post is possibly right. These cars are famous for that one.