Annoying loud hum

We just took delivery of a refurbished Gateway ML6728 notebook. (TigerDirect)

It produces an annoyingly loud speaker hum (that sounds not unlike an AC hum) whenever the computer is 1. externally powered and 2. when connected to any any receiver or amplifier. The noise does not appear when headphones are used or powered speakers are connected. The hum disappears from the speakers whenever the AC/DC power is disconnected.

The computer was purchased for the specific purpose of providing music at school sporting events, so it's imperative that a solution is found.

We would appreciate a workaround for this issue.

 

This is a followup to my post regarding an "annoying hum" emitting from loudspeakers connected to either an amplifier or a receiver. The occurrence of the issue was only while the Gateway Notebook was plugged into an AC outlet (but not while operating only on battery.)

I had the entire unit tested by an electrical engineer and he found the "hum" (and some "crackling" sounds) were being caused by an AC leakage through the AC/DC adapter (in my case the power supply cord is a Gateway ADP-65HB BB AC/DC adapter.) The hum was, in fact, an AC hum.

He advised that I use the AC/DC adapter only sparingly until replaced by a non-defective unit because the Alternating Current can shorten the life of the battery and has the potential to damage the processor(s) and the screen.

The engineer gave me a little tip to readily check for AC leakage. If your laptop has a metal case: While the laptop is connected to the adapter, very lightly move the palm of your hand across the surface near the touchpad and see if you feel a slight tingling or similar sensation while doing so. Then unplug the adapter and repeat the above step. If the tingling sensation is gone when this second step is performed, your adapter is most likely leaking AC into the machine and the adapter should be replaced.