There could be a number of reasons, but most times that has happened to me, the tires were under-inflated. Check your tires.
Also check the tightness of the belt that drives the power steering pump, it may be loose and slipping causing the squeal.
Could be the power steering pump.
high pitched squeal from left front wheel does not stop when I apply the brakes. It will stop when I turn the steering wheel even a little bit but returns when I straighten out again
It sounds like the ower steering pump might be starting to go
power steering
Low Power Steering Fluid
Low steering fluid can cause a steering wheel to whine when turning. A worn steering belt can also cause a steering wheel to whine when a car is being turned.
Loose or worn steering parts, suspension worn, tires out of balance.
If the squeal is when you turn the steering wheel (especially when you are parking it) could be the rear differential. Our 2004 Rendezvous rd had to be replaced 3 times. Every time we have had a squealing sound associated with it. I'm not sure what year yours is, but your Buick dealer should be able to check it out for you and you may have it replaced for free if it is covered under your warranty.
u joint
steering wheel in locked positon try moving it a little either way and the key should turn. The steering wheel has a locking device that wont let the key turn if the wheel is not free or you have the Wrong key
Horn ring and/or steering wheel hub is bent.
It is the power steering pump working hard and making the belt slip, try not holding the steering wheel all the way till it stops or backing off a little when it is turned as far as it can go.