Thomas Mann Jr

I want to determine what causes a knock in a car's engine. I am familiar with an engine that knocks based on past experiences, and am able to recognize them. Approximately, three weeks ago, I heard a knock in the engine of my hybrid car's engine. I called it to the attention of my wife. She heard it. She drove it for about one week and it continued to knock. She took it to a dealer, the oil was changed, and brought it home. The knock returned. I then took it to a second dealership to learn the cause of the knock. That dealership kept the car for three days. I picked it up and was advised that the dealership could not confirm the knock. I drove it home, but did not hear the knock. I then took to another mechanic for further confirmation. That mechanic said there is no knock.

So, my questions is: " What is the best plausible explanation for what happened to the knock?" I heard clearly on several occasions, and so did my wife. What happened to the knock?

Please give your response ad advise of your charges.

Tom Mann