Strange clunk

[AirSpeed]

I recently noticed a clunking sound coming from the front drivers side of my 72 GMC 4X4, it's a sharp sounding clunk, not dull, it sounds like a hard piece of steel hitting the frame, I can hear it when I'm going slow and hit a small bump or uneven road. I can feel it in the floorboard with my feet. I spent more than an hour under the truck tightening everything up. Nothing was loose, the tie rods ends are all good, I can't find anything that could make this sound. I jacked up the axle and tried to recreate the sound with no luck, then I jacked up the frame and tried moving everything around that way and can't find anything! The truck drives nice and smooth, there is no vibrations, everything seems fine! I opened the hood and checked out everything under there, everything is tight!
What could I possibly be overlooking? This is driving me nuts! Thanks for any suggestions!
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