Diagnose if I have a bad main bearing.

[64dartgt]

My mechanic says my recently rebuilt engine has a bad bearing.  When it warms up it sounds like someone is thumping on the bottom of a plastic pail.  I took it to the guy who rebuilt the engine and he says it isn't the engine.  I took it to a mechanic he recommended...real old school guy with a lot of experience...and he said it might be the oil pickup banging on the pan or a bad fuel pump.

I did some research online and it seems that I should be able to do some diagnosis with a timing light and pulling some plug wires.  The timing light should tell me whether the sound is on beat with the rpm (but I don't have a tach) but I don't understand quite how to do this.  I also saw that pulling the plug wires in sequence should make the noise go away when the wire for the potentially offending cylinder/bearing is not under as much load from firing.   I am concerned about whether or not I need to worry about where the spark will travel to if I pull the wire from the cap.  Alternately I can pull them from the plugs, but again...do I need to ground the wire or even put an extra plug in the wire and ground it.  This guy just does it while the engine is running using gloves and puller, but he also says this won't work for a main bearing.  It definitely doesn't sound like a rod bearing though.  Frequency is about every second at idle.

https://www.youtube.com/watch?v=JIOdS7XbHys

Thoughts?

Thanks in advance.

Bob

[ws23rt]

If the thumping noise happens about once a sec. that would be once every 15 revolutions of the engine at 900 rpm.

[RallyeMike]

The engine builder says is isn't the engine. So it continues to knock when its not running   

I don't have much to add other than the possibility that it is the oil pickup hitting the pan is about zero unless the crank throw is whacking the tube and its loose. Assuming we are talking about a stock type pickup, both it and the pan are fixed in position.

You are safe to pull the wires off the plugs with a glove.

[ACUDANUT]

A oil pick up tube is not going to cause thump noise.  Unless it is loose, which I highly doubt. Where can it go ?
How does it knock if the engine is off ?

[John_Kunkel]

I took it to the guy who rebuilt the engine and he says it isn't the engine.

Of course...what'd you expect him to say? 

[Challenger340]

Main
#1 change means squat.

Listen as you rev it lightly and then decel.... just as it rolls over the top of the rpm and starts decel.... more pronounced.

Main