I have a 70 F250 with a fe390 engine. Suddenly I get a rough idle followed a couple of days later by a distinct valve tick. My oil pressure gauge is running at the low end at idle. I pulled the valve covers and have no oil flow from the rocker arm holes. I started the engine and put a hose to my ear from each valve and about half of them are noisy to some extent. most of them can't be heard except by the hose method. What could have suddenly caused this lack of oil flow? The engine has less than 100,000 miles and is really clean inside. I know the prior owner and he changed oil like clockwork. I did have occasion to let the engine idle for long periods to charge the camper battery. Could this have caused irregular wear on the crank or cam bearings? I'm not familiar with this type of engine and have had no success in learning how the oil flows throughout the fe engine. Non of my manuals including the factory manual gives a decent description of the lubrication system. Any help is appreciated.