Subwoofers are a frequent home theater troublemaker. After a recent audio overhaul, my subwoofer started to wheeze constantly, producing a humming sound which was significantly louder than the audio it was putting out. The additional sound was distracting enough to make me unplug the subwoofer altogether for a while. Happily, I was able to fix the problem.
The first step was to determine if it was the cable or the subwoofer creating the sound. Most often, it's either the coaxial cable or the power plug. Common Coaxial Issues
An incompatible coaxial cablewith grounded devices can create unnecessary noise. The best way to test if it's causing issues is to simply turn on the system and remove the coaxial cable and reattach. If the sound remains, try another cable. If a new cable doesn't help, compare the audio output from the cable box and another device. If the cable box produces more noise, then call your service provider and describe the issue. It's likely a grounding issue with the provider. If the sound remains or seems to change very little, it's more likely a power issue. Common Power Issues
If the coaxial cable is functioning properly, the next step is to review the power source. I've heard others claim a cheap power strip is a common culprit. However, it's more likely two separate three-prong plugs in the home entertainment system are competing for the ground.
Here's what I mean: most subwoofers are grounded, which means they have three prongs versus two. Many amps are also three-pronged, so if two are plugged in at the same time, they are competing for the ground. This is called a "ground loop," because the two are not ever arriving at one ground.
On the back of the subwoofer and amp casings, there may be a switch to define the ground or defer to another ground - if that's the case, make the subwoofer the ground. If this option is not available, buy a "ground loop isolator" and plug it into the subwoofer. Define the ground and the hum should disappear.
So what happened with my subwoofer? I actually have a pretty old model, so I determined I needed a ground isolator, and set the subwoofer as the ground. Humming and wheezing, all gone!(Photo Credit: Johan's Ultimate Home Theater Hack, Stinger's Audio; Model SGN20 and Gino Audio)