Buzz sound in garageband with USB mic

Hello,

I would like to request help please - I´m desperate. In Garageband I have constant background noise - a faint high pitch buzz sound at a rate of approximately 130 beats per minute (reminds me slightly of an alarm clock). Recording is not possible.

I use an iMac (processor 1.4 GHz Intel Core 1.5 Memory 8GB MHz DDR3, OSX 10.9.4) and Garageband (version 10.1.1). When I connect my microphone (Shure SM58) via USB cable (preferences—>Audio/MIDI selecting ´Input Device´as ´USB PnP Sound Device´(SM58) and leaving ´Ouput Device´ as ´System setting´) the buzz occurs. It is worse when the input volume is higher but it is alway sthere regardless of level (plus I don´t here the mic if turned too much down). If I don't put ´Output Device´ as ´system setting´ I get sound no matter what. If I leave the ´Input Device´ as ´System setting´ the iMac uses its in-built microphone with no problems. But I want to use the SM58 for recording purposes…

What I have tried:

—I connected my mic and same cable to laptop (HP intel core 1.5) and loaded up Audacity (free audio recording program) and it worked fine. No buzz. Power cable plugged into laptop or not.

—I have moved my iMac to different power sockets throughout the house (including the same one I used with the laptop), but get the same buzz issue everywhere. More mid in the equaliser definitely makes it worse.

I love the garageband sounds (i.e turning my piano into a Steinway Grand Piano). If any equipment I could buy would stop the problem I´m more than willing to buy it. I get the feeling the iMac is simply not good enough? (Im new to Macs)

Thanks for your time,

Andrew