cassette to mp3

I have been up and running with cassette to iMac for a few weeks now. I even upgraded to stereo with a behringer USB sound card. I use garage band for the software. The results have been sounding great, however, I need help, I'm trying to tackle a minor issue. Every few minutes, a random audible static sound that I hear during the process is also recorded onto the mp3 result. I've eliminated all electrical devices in the area that could be transmitting something picked up by something in the loop by turning things off and unplugging them but all to no avail. Could this be a result of needing to ground the USB sound card to the cassette deck? Could the new sound card itself be transmitting something? Thanks for any advice. Mark

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