nook
+1y
Without actually hearing the noise its hard to be specific but heres a few ideas , is the oil level full? is the noise consistent, a steady knock knock? possibly a rod but hopefully not. The oil pump, gear driven off the front of the crank, pretty foolproof but do occasionally go bad. They can be checked with feeler gauges, the gears inside have specific clearances to the housing, the timing chain mounting surface where the pump mounts should be smooth and flat, any grooves can cause loss of oil pressure. The front seal leaking is common on the 20/22R engines and can leak more than it appears as the fan blows the oil back underneath making a mess. The crank pully is technically a harmonic balancer, the early engines were a solid cast unit, the later engines had a rubber banded balancer to better absorb harmonics, and they do come apart once in awhile, also a crack in the keyway or the bolt backs off allowing it to move around making noises.
Coming down the hill on compression unloads the engine and the vacuum lifts the oil to the top end of the engine, thats why I asked is it full, a loss of oil pressure could allow the timing chain tensioner to loose tension and sometimes they can stick in letting the chain flog around in the cover, they will really rattle then, also if the chain is getting old it could have broken the drivers side tensioner rail, if so it will eat into the timing cover in short order making a real mess of water/antifreeze into the engine.
Popping off the valve cover you can look down the cover at the rail condition and also the chain tension. Good luck.