2005 Cadillac STS Noise at low speed

Have you checked the power steering fluid level, if so, was it low? Does this noise occur when stationary and engine speed is raised? If it does, you may be able to remove the serpentine belt and run it to see if the noise is still present, this would isolate it to a belt driven accessory and narrow the possibilities. There also is a bulletin that may match the description of your concern, it is;
#06-06-01-018: Information on Very Faint Engine Whine Noise at Speeds of 0-48 KM/H (0-30 MPH) at 1000-2500 RPM on 3.6L LY7 Engine - (Aug 2, 2006)

Subject: Information on Very Faint Engine Whine Noise At Speeds of 0-48 KM/H (0-30 MPH) At 1000-2500 RPM On 3.6L LY7 Engine

Models: 2004-2007 Buick Rendezvous

2005-2007 Buick Allure (Canada Only), LaCrosse

2004-2007 Cadillac CTS, SRX

2005-2007 Cadillac STS

with 3.6L V6 Engine (VIN 7 - RPO LY7)

A very faint engine whine-type noise heard at speeds of 0-48 km/h (0-30 mph) at 1000-2500 RPM may be caused by the primary camshaft drive chain. This type of noise may be amplified by the vehicle's body structure. The noise may be present at idle (hot or cold) and, in some vehicles, this noise may be louder.

GM Powertrain Engineering states that this type of noise is characteristic and/or normal for this engine. No abnormal engine wear or internal damage will result due to this noise condition.

If an engine whine, whistle or ringing-type noise is heard from the front of the engine and increases in intensity as the engine RPM increases, refer to Corporate Bulletin Number 05-06-01-013B.

An engine noise primarily around 1600 RPM that can be described as a howl will typically be coming from the exhaust system. This is also considered a characteristic and/or normal type of noise.

I will also include the bulletin referenced above titled whine, whistle, ringing, etc.
#05-06-01-013C: Whine, Whistle, Ringing Type Noise Heard From Front of Engine (Replace Crankshaft Harmonic Balancer) - (Aug 16, 2006)

Subject: Whine, Whistle, Ringing Type Noise Heard From Front of Engine (Replace Crankshaft Harmonic Balancer)

Models: 2004-2005 Cadillac CTS, SRX

2005 Cadillac STS

with 3.6L Engine (VIN 7 -- RPO LY7) and

without Heavy Duty Cooling (RPO V03/V92)

This bulletin is being revised to add additional information in the Condition statement. Please discard Corporate Bulletin Number 05-06-01-013B (Section 06 - Engine Mechanical).

Condition
Some customers may comment on a whine, whistle, or ringing type noise from the front of the engine that increases in intensity as engine RPM increases. This noise is most audible standing in front of the vehicle with the hood opened.

A very faint engine whine noise heard at speeds of 0-48 km/h (0-30 mph) at 1000-2500 RPM may be caused by the primary camshaft drive chain. Refer to Corporate Bulletin Number 06-06-01-018 for more information about this type of noise.

Cause
This condition may be caused may by the harmonic balancer.