Noisy hydraulic lifters were a problem when these engines were new, and 50+ years of use hasn't improved the situation any. Whenever an engine stops, at least a few valves are open, and the constant push of the valve spring against the lifter causes it to leak down and bottom out. It's supposed to refill as soon as the engine is restarted, and oil pressure builds up, but sometimes it takes a little while for that to happen. We used to run a thin solvent-type oil additive such as Bardahl or Wynn's Friction Proofing to clean out the check valves and help the lifters function right, but I'm not sure if those products are even still on the market. Add about a pint of Diesel fuel or Mineral Spirits to the crankcase and run the engine at fast idle for 10-15 minutes before an oil change, and the same cleaning effect will happen. Just don't drive the vehicle and load the bearings while the solvent is onboard.
Jerry
