Please keep in mind that your '90 uses the miserable, adjustable Crankshaft Position Sensor (CPS), so it will have to be aligned (at least verified) with the new harmonic balancer.
Our '92 3300 (the odd, little brother of the 3800) has needed at least 3 harmonic balancers (one warranty failure too) between 30K and 186K at the GM dealer. When I changed the timing chain, I found the CPS had been 'shaved' from the vanes of the harmonic balancer, I'd guess from the warranty swap. It was always the same SOB that worked on this car for the previous owner, and the work was pathetic, but premium priced. My favorite was the stripped O2 sensor threads in the exhaust manifold.

I have a jpeg video but I cannot upload... it does however sound very similar. I want to properly remove the bolt from the balancer, not the unsafe practice of wedging a breaker bar like some online videos show (yikes)

I have an electric impact wrench which I'm hoping is strong enough. On this particular model, 1990 3.8L can anyone point me in the right direction for wedging the flywheel so I can do that? ie cover location...

Thanks! (i'll work on the video upload)

I dont really feel a wobble...

Here is a link to video via FB Balancer noise? Seems the same as the other video post

I'd suggest you discover first where the noise is from before arbitrarily replacing the balancer. It'* easy to do. Take the belt off and check each pulley for play. Any play and you might have found your issue, or another issue that would have surprised you later.

If I had to guess based on the audio in the video, I'd suspect a water pump or alternator bearing. Since you've confirmed that the harmonic balancer doesn't wobble, it'* be good to check for other suspects first.

Don't forget to do the screwdriver/stethoscope test discussed earlier. That can be very revealing. If you are comfortable with it you can do it on a running engine, just keep in mind that if your screwdriver/stethoscope probe slips you might have major problems.

FWIW it'd be easy to spin the (belt-removed) alternator with a drill then listen to it as it is spinning.

What was the idle speed in your video? It sounded high.

Thanks for insight... new water pump, alternator is 2 years old, idler bearing is good...

BTW just started a cold motor so about 1200 rpm?

To confirm a harmonic balancer failure, you can grab the belt, and try and move backwards and forwards, essentially trying to drive the accessories as the engine would. If the balancer is good, you likely won't be able to budge it. If the balancer is bad, it will move back and forth some.

I recommend gloves when trying to do this.

In my case, I was able to move the entire belt path about an inch. Essentially, when the balancer fails, there is nothing locking the crank portion to the drive belt portion. It'* designed as such that it doesn't rotate on itself but a few degrees, but the noise is essentially it hammering itself back and forth.

Could it be that the bolts on the water pump pulley are not tight?

Shot in the dark, but would feel silly not asking if that were the problem.

Otherwise: Do what rjolly87 said ^^^^^^

It was in fact the harmonic balancer. Once removed from the vehicle you can see the breakdown of the hardened rubber material that is part of the balancer. No knock anymore. Thanks everyone for your input!