[projectRDM]

I love when people randomly guess dumb shit for the hell of it.

To the ones who quickly jumped on the OP's suggestion, you're morons. The tank sits a good three inches below the chassis floorpan in most places. Even at it's closest it's maybe an inch away. There is no possible way the bolts contacted anything unless you welded another 60-80mm to them first. The noise you heard is the layer of undercoating applied on the floorpan that sits flush against the bolt end, you've just pushed it out a bit by threading the bolt back in. There's nothing to worry about.