Valve Lifter noise under weird conditions

4.2 l Old Bravada inline 6. Car develops valve lifter type noise only after about 200 miles on a trip. Noise persists.
After arriving at destination and sitting overnight, it’s gone. On the return trip it appears again after about 200 miles,
and goes away again just as before. It’s a light noise, not a knock. Car is full of clean new oil (Castrol high mileage) of the proper weight. Any ideas?

A couple of ideas.

Get the running oil pressure checked ASAP. It may be getting low enough to make the valves clatter, but not enough to turn on the ‘oil’ light. A lot of those light on sensors don’t activate until a very low 6-8 PSI, which may not be enough to keep the hydraulic valve lifters pumped up.

Also, it could be pre-ignition. If the knock sensor is failing, the computer may not know to retard the timing to prevent the clatter.

The oil thing might be relevant, since the noise takes a long drive to develop. This so-called high mileage oil has more additives and less oil. That’s what he was using–the Castrol version.

Also change your oil filter. It may be partially clogged and the bypass may not be allowing enough oil flow either.