I have a 2WD 1500 Suburban that just started having a medium/high pitch hum coming from under the hood and can be heard from the driver seat. This hum is constant and does not change pitch based on engine revs. Under load (accelerating up a freeway onramp) the noise seems to go away but returns upon reaching cruising speed.

I know these Subs have naturally noisy fuel pumps (mine included) but this new noise started only a few days ago.

I have done things like turn the a/c off to see if the noise went away but it didn't. Turning the steering wheel to see if it related to the power steering pump had no effect.

Any help would be great!

If the noise is coming from under the hood,I would check the alternator and waterpump.
You can do this by using a long screwdriver as a stethiscope. Hold the pointed end on the body of the alt or pump and put your ear to the handle.Remember, put the screwdriver on the body of the alt or pump...NOT on the pulley!!!!!! While the engine is running you can hear and feel where the noise is coming from.

Both the water pump and alternator have been replaced within the past 4 months so I wouldn't think those would be the source. Besides, wouldn't the pitch or intensity of the hum change with the engine revs? This hum really sounds more like an electric pump that stays at the same speed regardless of what the engine is doing.

Check to see if the ABS pump is running. It is located under the drivers side of the truck directly underneath the driver.

I'm scratching my head on this one...main reason is I can't hear the noise. Does it only occur when the engine is running or will it make the noise if you turn the key to the run position?

As far as parts being replaced...new parts go bad too.
One thing I've learned in this business is just when you think you've seen it all....... :eek7:

The noise is only present while the engine is running. I put the ignition in the on position the other day and no noise.

The other thing is that when the engine is turned off, the noise continues for about 1 to 2 seconds. It sounds as if it relates to pressure and then when the engine is shut down, it takes a second or two for the pressure to release or reduce enough to stop making the noise.

The noise is more like a whine than a hum really. The main thing is that it's not super loud. If the climate controls are on medium or higher, it drowns out the noise completely.