Hydraulic Lifter Noise

Do your 235/261 lifters start making noise when the engine is run at highway speeds then quiet down at lower speeds? In most cases there is a simple fix.
The shop manual gives a detailed description on how to fill hydraulic lifters before installation, but most people skip this step, or think soaking them in a can of oil is enough.
Most noisy hydraulic lifters can be attributed to air trapped in the lifter. Once trapped it stays there.
In most but not all cases the noisy lifters are aftermarket.
Most aftermarket lifters have a hole in the pushrod seat, (this is the area the pushrod sits in).
By grinding a small groove in the bottom of the pushrod from the center out to the edge air is allowed to escape.
The air will come out from the hole in the pushrod seat, and through the groove you cut in the bottom of the pushrod.
This groove can be less than 0.010" of an inch deep, enough to allow air to escape, though the depth is not critical.
Chevrolet did this very thing in their 1950-52 engines from the factory.
This repair has worked well for me and others I've helped.