The odds are you are hearing power line noise, which is caused by poorly maintained power lines. It is becoming even more common these days as utilities slash their work forces to "position" themselves for de-regulation, and linemen have a full time job just to keep the lights on. If your radio can run on batteries, try unplugging it from the wall. If the noise went away or lessened significantly, you're in luck, the noise was coming into the radio through the power supply, and you can install a brute force filter (Radio Shack sells them), which should help.
Usually, though, the noise is entering your receiver through the antenna. This is definitely the power company's fault. Unfortunately, even though they know better, many power company officials try to make you back down, hoping you'll give up and go away. They claim the signal is coming from someplace else, that they are not responsible for shortwave problems, or simply that there is nothing they can do. The power company is obliged to fix the problem, no matter what they tell you. If you (or they) doubt that, and they repeatedly refuse to fix the problem, contact the Federal Communications Commission. However, many times the power company is doing the best they can, but still take months to resolve the problem. In this case, or if you just don't feel like fighting them, get an MFJ noise canceller. These are not cheap, and they take a little practice to learn to use, but they will work wonders on power line noise.
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