IMac 24 radio frequency interferance during sleep mode

I have noticed that my iMac 24 produces radio frequency interference which I can hear on the AM broadcast band when I am listening to 1370 AM while using my computer.
When I am using the computer there is no noticeable interference on the AM tuner which is about 10 feet away from the iMac but as soon as the computer enters sleep mode, I can hear a constant tone on the radio about 1760 HZ along with the rest of the audio content normally broadcast. The tone is not overwhelming but is noticeable enough and annoying.
This 1760 HZ tone was determined as my closest approximation by comparing it with a piano key note when I play various notes for comparison purposes and it appears to closely correspond to key number 73 on a standard piano keyboard which is A6 or 1760 HZ.
I would like to know if this is a commonly observed problem and how to avoid it if possible. The iMac 24 I am using is a model A1225, part number MB418LL/A, serial number W89243Y00TG.
As far as I can recall, this has been true since I began using this iMac and did not happen after having the computer for a period of time and then appear suddenly. I just failed to connect the annoying tone to the sleep mode of the computer until recently when it was clear that as soon as the computer came out of sleep mode that the tone on the radio disappeared until sleep returned.

First - you aren't disconnecting any device while your computer is asleep are you? This can cause your problem.
Assuming this isn't the cause, let's do an SMC reset. Shut down the computer. Unplug all cables from the computer, including the power cord and any display cables. Wait at least 15 seconds. Plug the power cord back in, making sure the power button is not being pressed at the time. Then reconnect your keyboard and mouse to the computer. Press the power button on the back to start up your computer. Does it now sleep and awake?
If not srojtas is right that one of your peripherals may be the culprit but rather than disconnecting one at a time I recommend that you disconnect everything but your ethernet cable (assuming you have one) and your USB mouse and keyboard. Create a new account and set the computer so that you have to choose which account to log into.
Restart and log in directly into the new account. If it sleeps and wakes up now you know that the issue is your account or one of your devices. One at a time, connect (and install support software for) a device. Does the computer wake and sleep? Move on to the next. You may find the culprit along the way. Removing it may or may not let the computer sleep depending on whether it is the device itself or the software.
BTW - the device at fault could be a USB or FireWire hub.

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    I normally only have speakers set up to my computer and a wired keyboard. The speakers are powered separately. I would assume that at the Apple Store only the power cord and wired keyboard are connected.
    What is the Power Supply Position? And what does it do?
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