'66 Mustang?- Nice one. Firstly, I'm assuming the motor has hydraulic lifters (not "old school" to the point of having mechanical ones, is it?). Faulty Hyd. lifters usually make an intermittant "ticking" type sound, usually at first start up and generally getting worse as the oil heats up and the lifter leaks down- never heard a rattle sound from them. I had an XP falcon with 170 ci that had a worn (disintegrated) cam lobe and it made a clacking sound at idle which got louder with revs - the most obvious symptoms being an idle that no amount of tuning would smooth out and flat spots through the rev range. You can check for cam lobe lift by rotating the engine and measuring the rocker arm/ valve spring travel (loosen or remove spark plugs first so you dont have to fight compression) It sounds obvious, but have you checked your ignition timing? Pre-ignition makes a rattling sound- usually apparent under load but if your timing is really advanced also at Idle- worse in older High comp type engines which run on modern fuels (no lead) I have to run octane booster in my Clevelands to avoid this or retard the timing and ruin performance. It's possible you have a bent pushrod- with the rocker cover removed rotate the engine untill there is no load on the rocker you're checking, you should then be able to rotate the pushrod freely with your thumb/ forefinger and if the pushrod appears to "run out" then chances are it's bent, valves + springs + guides etc could also be possible causes of a "rattle" I guess. Your rocker/pushrod is meant to squirt oil- thats how fulcrums etc are lubricated by the way. Good luck with tracking down the cause. Mick.
