Maybe record it for us.

The rattle is coming from the front of the motor. When I start the car it has a light knock. Then when I take off as my rpm'* climb to change gears it rattles. Like something loose. Then it stops and when I accelerate again it rattles again. In a short spurt and stops. I have never heard a car make that sound. I am not sure how to record the sound for you, but I will try.
By the way, I really appreciate yall'* input on this. Thank you!

I don't know the clearance on the rod and main bearing but I will find out.

I assume you pulled the original harmonic balancer with the proper puller and then reused it? Have you checked to see if you can rotate the outer ring on the balancer back & forth?

Doe the engine use a timing chain tensioner? If so it may need attention.

Spring timing chain tensioner on a 3800 Series 2.

Yes. The balancer was pulled properly and it was replaced with new one.

My engine does have a tensioner arm on each side of the chain. Replaced chain and gears.

Are those hydraulic tensioners? If so, did you check them before installing them? Seen many that were bad out of the box.

They are spring tensioners..