1992. Why is it rattling at warm idle in lower motor by right side peg?

Do you have an engine light on? Take it to an auto supply store. They will usually check it free.

With a bottom-end rebuild, I assume the engine was removed from the frame?

Maybe an exhaust cover is rattling.
Check the mount clamps, and see if any are riding/making contact with something else.

Once I had a right hand muffler that rattled ONLY at idle.
The rubber block (that goes into the lower saddlebag support rail) had "shrunk" and allowed extra movement of the rear of the exhaust system.
Funny how it was only at idle though..............

on a cold start car will idle fast till it warms up then will drop to about 1000 rpm, try increacing idle speed so when it warms and drops it should stay idleing

Have you changed the fuel filter?

Idle Air Control or a vaccum leak

Remove the glove box.Then remove the glove box lower frame.The blower motor will be on the
right hand side under the dash.Remove three 10 mm screws.After removing the screws pull the
blower motor forward and up.There will be three harnesses connected in the lower right side
of the blower.

this is easy one if you put the a/c on do you hear the rattle if yes then shut off the a/c if the noise go away then it is the compressor

Vacuum leak.Listen for hissing around motor. check spark plugs.Idle positioning sensor. Idle Air Valve. IAC clogged dirty/clean an inspect for air leaks. I have an Nissan 300zx an they are located on back side of engine on driver side. It runs into the Pentium Intake between two 3/4 in rubber hoses/Driverside--Change plugs an wires.

the idle air motor in the throttle body is probably not working right it lets more air in the intake when its cold and is supposed to shut off when it warms up if its not shuttin off then it is probably not closing

Having the harmonic balancer replaced today on my 94 topaz, rubber has broke in balancer