I did a few years setting up music studios and that kind of stuff so maybe I can help.

with that type of problem the thing to do is check your connections first. pull all (yes all) of your connections all the way out and make sure you push them all the way back in. listen for any signs that a cable or jack is going bad. if your system is older then you should take note of any cables that seem fragile.

next you want to check all your audio settings. this would include any software, driverware, or hardware that may be in your audio chain. may sound like its too easy but sometime an eq knob somewhere is all the way down/up.

if you don't eliminate the problem those two chains then you start to trace the entire chain from the power all the way to the speaker. also ask yourself if it sounded right at some point and what changes have happened since then. remember that everything in your home that requires outlet power is all pretty much connected at some point. sometimes i have had to unplug stuff in other rooms looking for a buzz.

I've had a guys calling me week after week about a buzz that only happened in the studio "sometimes". how about those somtimes were always the times he would plug his cell phone charger in. same thing happens with laptops and other wall wart and line lump power supplies. sometimes a power conditor (not just a power strip) can help filter out the crap that comes in but there are some cases that have been difficult to solve.

probably not a big issue and there are options to get you where you want to be. check it out and keep us posted.