I attached a file of the engine at idle. It is hard to hear both in real life, and harder in the recording. I can hear it in the recording, but it might not be noticeable with different speakers. There is a quiet knocking noise that is easier to hear from under the car or in the wheel well. Sometimes it seems to be rhythmic with a cylinder, other times it seems slightly more random. Maybe its two different things? It gets faster with revs, and goes away. I can hear it both hot and cold, and I think it may be louder when cold though. I rebuilt the engine and im not sure if this started as soon as I finished or recently. Its hard to notice and I only noticed because I am paranoid about something wrong with the engine. Maybe it is not even the engine as I had an old jeep with a cracked flex plate that made the same kind of noise but way louder.

 

It does it stone cold? First step is do exhaust leak test. Slipped liners are almost never there stone cold, usually start at about 190 degrees.

 

Both stone cold and when warm. It seems louder when cold though. I know I have an exhaust leak, but how could this be related to it? I got a better recording of the sound. It seems like its eliminating from the back of the engine at the bottom and possibly the front of the transmission. I put my phone under the car by the back of the oil pan record it.

 

Very difficult to diagnose any noise with an exhaust leak present, fix that first.

 

32 knocks in 5 seconds equals around 384 per minute. So, about 1/2 of RPM (assuming 750 RPM), meaning it is likely valve train noise, but fix the exhaust leaks first as mentioned above.

 

if you don't listen is there really a knock ?