the strangest thing, i have had this happen about 2 or 3 times in the past 3years, i bought a 1973 440 from a buddy of mine, he sd the heads were rebuilt with harden seals for unleaded gas, he sd the bottom end looked great, he checked out everything and it was fine, this is an old motor home motor with low original miles, all the bottom end is original, the cam, lifters, intake carb and headers been added, sometimes when i crank the motor, it sounds like a rod knocking or a spun bearing, the more i rev up the motor the sound gets better, however at idle it really sounds like the motor is junk, it last for about 5mins then goes away, and i may not hear it again for 6mths, i have been restoring the car and have not had the car on the road for 3years, i have made several trips around and around the block, i usually crank the car at least 1 every 2weeks and let run to normal temp, great oil pressure, running at idle has over 70 at cranking and about 60when idling hot, during the sound, the oil pressure is at high reading, has anybody ever heard of this?