If the 123 is similar to a 126, there is a metal shield that covers the drive shaft and is bolted to the body. That shield would "rattle" like thin metal. Most other things would clunk. The exhaust is the other suspect. Make sure the down tube is clear and that all 4 rubber exhaust hangers in the rear are intact.

I've added exhaust hangers to the traveling in-trunk parts list because one didn't last long and dropped to the ground. Bailing wire is also in the kit - now.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.