I think I can here ya KNOCKING!

So I've been working on getting my 65 Barracuda up and running and of course I've run into several issue along the way, and the people on this great forum have really help me out so far.

Now I have another one.

What I have is a recently reworked over '73 340 with solid lift cam that I got with the car, it's connected 904 Trans with a high stall conerter. I've been having trouble getting to idle right, but other than that it's been running really strong. That is until the other day. I took it out for test drive and it was running great, but it then started to have a mild rapid knocking noise! I tried pulling the spark plug wire test on each cylinder one at a time to see if the knock went away but it didn't. I went ahead and pulled the valve covers and even the intake and the valve train and cam and lifters all look good. Is there some other tricks I can use to try and determine if it is indeed a rod knocking or maybe something else without tearing the engine apart 1st? I think I can rule out "piston slap" because the knock is still there even after the engine gets good and warmed up.