You likely have some worn bearings in the engine. Pressure should be higher than 30psi when cold, and around 15 when idling and warmed up.
Does this engine have hydraulic lifters? If so, you are hearing them because there is not enough oil pressure to pump them up at idle when it is hot.
Afraid it's time for a rebuild.
As already said, Fatscos is a good place to get the parts for the hydramatic.
