So I figured out that the noise I was chasing was not in the gas tank. I thought it was coming from stuff in the rear quarter-panels. I bought a
wireless endoscope to help find the stuff. I found this stuff in the quarter-panels and in the space between the door frame and the rear fender well:
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A 9/16 socket, a quarter, two nickels, two rocks, and two nuts. I know I probably dropped the nuts while installing the aluminum trim, the rest of the stuff had been in there for a long time. The change has epoxy on it, I guess one of the previous owners tried to glue them behind rust holes. Here's a few pics from the endoscope:
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The first one shows a nickel still stuck to the sheet metal. The second pic shows me extracting one of the rocks with a piece of hose attached to the endoscope. The other end of the hose was attached to my shop-vac.
Getting this stuff out fixed the sounds of stuff moving when braking or cornering, It did not fix the clunk from the passenger side when driving over bumps. I checked and tightened everything in the back, snugged up a bumperette and one of the gas tank bolts. The clunk was still there. The I started tapping on everything in the back with a rubber mallet. It was all solid except for the right leaf spring. It rattled a little around the tail end where the main leaf wraps around the end and returns. I added a piece of gasket cork and the noise stopped. Here's a pic:
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Now that I'm looking at the pic, maybe I should have just tightened the u-bolt on the left side of the pic?