air entrapment would manifest itself as a moaning sound from the power steering pump specially when the control valve operates (steering being turned)

a squeal would be more a loose belt or severely worn out fluid in it in arctic winter causing the pump to lock or a fully clogged filter in the steering oil reservoir

I dont think I have seen an air trapped system in a W124, seen quite a few on W210 (and other rack model MB)

I use the GM procedure to remove air from the rack and pinion MB cars, I apply a vacuum of 20 inches to the pump reservoir and pull out the air, cycling the steering lock to lock with engine off and tires in the air pulls out the air bubbles.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)