I had this problem in an amp within the speaker. I swore up and down it was the transformer. Turned out it was a faulty load (bad solder joint) that powered the digital electronics. One drop of solder fixed it.

Although Vinces warning was good and valid don't let it scare you from taking a peak. It's probably less dangerous than opening the hood of your car while it's running. Common sense applies.

I would poke around using something like a long wooden spoon or paint stirrer while it's open and buzzing and you may get lucky it's just a tie wrap or cable vibrating against the transformer or a screw lose. If you're not the DIY handy man type then don't do it.

I work with fairly powerful lasers and 30,000 volt power supplies, those will give you a real kick pants. So I may be a little desensitized.

- Fiddler (IMHO)

If you think listening to music is fun, try learning an instrument.
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