Take a Very Close Look at the condition of the Ribs and Valleys of the Serpentine Belt. If you can see ANY unusual Fraying of the Cords or separation and Excessive Edge Wear of the Belt or inside the Grooves... you might suspect an
alignment problem with one or more Pulleys or Accessory Components. A Misaligned Belt is capable of making
Horrendous Noises from the
Whining, Squeaking and Chirping caused when the Belt passes akimbo Clockwise over and through the Grooves and onto the Smooth surfaces of the Pulleys.
Since you mentioned replacing the Harmonic Balancer... it would have been imperative to do the following:
(1) Used a New Harmonic Balancer Friction Washer placed onto the nose of the New Harmonic Balancer.
(2) Prior to beginning, First having Locked the Flex Plate rigidly in place through the Hole in the underside of the Rear of the Crankcase-Oil Pan. This works using a Breaker Bar and a 15mm Deep Socket placed over One of the Three Torque Converter Bolts
after removing the in-dwelling Black Plastic Plug and lining up the Socket and the Bolt Head inside of that Hole.
(3) Then after having installed a Brand New TTY Crankshaft Bolt and applying 110 Foot Pounds of Torque, it would've helped to have Painted a White Vertical Line across the Hex Bolt face running from 12:00 to the 6:00 Position, marking the Bolt Face as a reference point.
(4) This should've been followed up by imparting the Additional Final Torque To Yield Force necessary to get the Harmonic Balancer - Serpentine Pulley TTY Bolt Face Rotated Clockwise just enough to Invert the 12:00 with the 6:00 Positions.
This activity is NOT Trivial since the HB -Serpentine Pulley is NOT held directly onto the Crankshaft via the 'Old Style' Slotted Woodruff Key.
Even though the Featured GM Vehicle in this Video covers these related issues on the GM V8 Engine ... the principals and concerns you have for your GM Atlas 4.2L I6 LL8 Engine Noisy Belt and Pulley Alignment issues are
Identical.
It cannot be overstated that if the Power Steering Pump Serpentine Grooved Pulley is Incompletely Installed on the PS Pump Shaft, the Proper Alignment of the Serpentine Belt between the Crankshaft and all of the Accessory Pulleys thus becomes Impossible: