1996 Chevy Tahoe changing engines

Ok, well ill give you a few things that can cause a knocking sound like you described and are relatively easy to check, and probably save you the trouble of the new engine.

I would eliminate the valves clicking because sounds like you got sufficent oil.

A torque convertor that is loose on the flex plate or a cracked flex plate will cause a knockiing sound like that, usually mistaken for a main bearing going. Its normally most noticable in idle when speed is low and no engine load.

Next thign to do is ground out one cylinder at a time and see if the noise stops. Do this by putting a piece of vaccum hose in the spark plug wire and on the spark plug then grounding it out with a grounded test light, if noise stops when one cylinder is disabled it could be a piston pin that knocks, it also usually gives off a double knock sound.

I would eliminate timing chain noise as if the vehicles chain is worn that badly your vehicle will be running poorly. With main and rod bearing noise it depends on speed and engien load the noise it will make so I dont think that would be the problem.

If you have a heat riser remove the vaccum line from it and remove the drive belt, does noise stop when this is done?

Are you saying the noise stops when you ground out a cylinder? Which one?

Take the drivebelt completely off and see if the noise still continues, a defective belt can cause a knocking type noise and is often mistaken as something much more serious.

And I would like to thank you ahead of time for the donation, it is very helpful!

Get back to me.

If it stops on a particuallar cylinder you can suspect a worn rod bearing on that cylinder or piston slap on that cylinder.

If it dont stop on any cylinders yoiu can suspect everything else I listed.

Id like to ask some more questions bout the sound

is it a clicking noise like clicking a pen?
A clanking noise such as tapping on metal?
Clunking like shutting a door
clatter, like marbles rolling around
knock, such as knockign on a door
or rattle type noise.

As I said above if the knocking stops then either you have a case of piston slap or rod bearing on that cylinder, even a hole in the piston!

Hook the groudn on a good engine ground on the block, touch the probe on the vaccum line you put on the spark plug tip and into the spark plug wire.

To ground it out simply touch th eprobe part of the test light on the vaccum line.

And since you said its a clacking noise, then the most common causes of that are, a worn pison pin, a broken piston even valve clearence that is out of specs will cause that noise due to the slack.

Does it attempt to crank? Crank slowly fastly? Or does it crank normally jsut not start?

When key is turned to start do the dash lights go out like normal or stay on? Turn on the headlights and attempt to crank it, anything?

I would double check all connections and grounds might have knocked one loose, make sure you hooked up wires properly also.

Sounds like a bad ignition switch to me.

Hmmm, sounds like the ignition switch is good since it clicked like that. Still could be a error with it but I wouldnt buy one till hte starter is replaced, also have the solenoid looked at also whle your there.

After 5 or 10 mins it starts making that noise not when cold?

If you shut it off and then restart it right away does it take 5-10 mins to reapear or does it start right away?

Are you planning on switching the engines out or no?

Maybe you can take the valve covers off see if theres any wear on the pushrods or lifters. Indicating a lack of oil to them or too little oil.