It would be very hard to cover up a knock. It'* more likely the engine was old and not able to handle getting on it for an extended time.

In the first stage, it will be real silent. Hard to pickup. Mine was the same way. Until you increased the RPM or put a load on it then you could hear it. When it really starts to go, you WILL know it. Just don't climb a hill at 3500 rpm. Things will come out of places they shouldn't. LOL.

Is it possilbe the previous owner used a heavier oil. Then when the new owner did an "italian cleanout" which made the oil hot and thin, caused the sound to come back?

The first sign is the oil pressure drops at idle to perhaps 20 psi or lower. After that, it'* all down hill real fast.

It is possible, because the day I bought it I had oil and filter changed at a shop so I can keep track of oil myself.

Also its funny you talk about load, the night I driving it in heavy rain I had 3 passengers which was the first time the car had some load.

Yup, same thing happened to mine last year. Heard a knock-knock-knock...oil was fine for awhile, but in the 90 summer heat, OP dropped and it was game over.

How much more driving this engine have left in it? 5miles?

Downhill with a hurricane behind you, ohhh, about 20-30 miles. Uphill with a tailwind, about 20-30 feet. Its not a precise science. It'* not a matter of if, its a matter of when.

Well I'm glad we don't have any hills here.

had a 3800 sound that way and it turned out to be the harmonic balancer just a thought