Try the many on-line auto parts sources, such as; www.RockAuto.com, www.autopartswarehouse.com, or www.partsgeek.com. They (partsgeek and RockAuto) list several air suspension air pumps from around $173.00 for your vehicle.

Try to navigate this link http://www.autopartswarehouse.com/search/ to see a replacement air pump for your vehicle.

Tow it to the dealer.

You car is able to display fault codes for the air suspension. It is called RSS data. Read this to learn how to do it. You cannot fix the car without knowing the fault data. Nobody can.

Need more information; where is the roaring / humming sound coming from - at the wheels or more towards the rear in the center? Is the noise localized - seems to come from one specific area? Several possibilities based on the information provided; faulty bearings - in axle or differential, low fluid level in rear differential / axle, faulty wheel bearings.

Typically, the spoked hub cap has a seperate key to unlock the hub cap (locked for security by the hub cap design). Usually you'll need the specific key to first unlock each hub cap to remove and access the lug nuts - the hubcap key may be located inside the glove compartment. The hub cap lock is usually located under the hub cap center cover or emblem.
Once the hub cap is removed, the vehicle can be raised on the corner to remove / change the tire. With the transmission in Park, and the vehicle on a fairly level surface, then raise the vehicle using the supplied scissor jack or a suitable hydraulic floor jack. Be sure to support the vehicle with a weight-rated jack stand - do not rely on the jack to support the vehicle.
Loosen and remove the wheel lug nuts, then remove the tire. The installation is the reverse of the removal.

Check the tire pressure and adjust slighty to firm up the ride - just do not exceed the tire pressure maimum rating on the sidewall of the tire. Most new vehicles suspension will tend to settle and break-in somewhat given time and mileage accumulated. Some GM vehicles have an adjustment to the struts and shocks to let the driver choose beteeen several settings.

You should verify first if there is still stored trouble code in your vehicle after you've delete/reset it. You can navigate this link http://www.extreme-check-engine-light-codes.com... to have an idea ho to extract stored trouble code in your vehicle without using a scan tool and there are some common codes meaning on that link for your reference.

Torque the wheel lug nuts to 100 foot - lbs. in a star pattern.

two thing mainly it could be, 1. bent rim or 2. you need you tires balanced " this is most likely the problem"

dealership repair required. codes set - inspect for brake drag damage, also, wheel speed sensors
tested, more.

Cannot state whether 20 inch rims will fit - it depends on the wheel backspacing, offset, tire size and other geometries. Try the many on-line auto tire retailers, such as; www.Tirerack.com or www.discountire.com for exact fitment and recommendations.

If the steering feels stiff and requires a lot of effort, the rack piston seal may be leaking pressure or the pump may not be producing adequate pressure. A loud whining noise often borders on squealing. It becomes loudest during the turn and then drops down. Many people would attribute this noise to a problem caused by a bad power steering pump. However, the problem mostly arises due to a loose power steering belt. When you take a turn, the belt travels through the steering wheel pump's pulley. When it has gone loose, it does not follow its normal trajectory over the pulley and slips, when the tension on it increases during a turn. That causes the whining noise when turning steering wheel. Another problem causing the whining might be worn pump bearings. You must check for both. To do that, you must open the hood and have a look at the power steering assembly. Check for the tension on the belt. If it's loose, just tightening the belt might solve the problem. If the whining persists even after tightening the belt, you will have to check for worn bearings.

Carquest lists & stocks the hoses for this model.