Noisy engine overheating and symptoms no longer apparent

When I started my car my engine made a loud rattling sound. I tuned it off and the rattle became intermittent and the temperature gauge came on after a couple of miles while driving it to the repair garage.
The garage said engine was beyond economic repair so I took it home and parked it up for about a four to five months while I looked for a reconditioned engine for it.
However, when I went back and started it again the noise had stopped and the engine no longer overheats. I have run it for over thirty minutes without any of the symptoms recurring.
I checked the oil and yep, the tell-tale signs that were there when I parked it up were still present. Yellow goo on oil filler cap and on the dipstick.
However, running the engine with the radiator cap off produces no visible bubbles although I did have to top-up the coolant reservoir which was low. After all the standing time the only thing that failed was the battery went flat, but was all good after I charged it up.
Can you explain why the engine problem is not occurring now, and what would cause it to disappear now? I am really puzzled.