This is usually related to record decks and earth connections. The two most common types are:

• A bad earth connection. Try to physically connect one end of a wire to the chasis of the record deck and the other end to a good earth source like a metal / unpainted radiator pipe.
  
• An earth loop which is caused by your record deck being earthed twice. Disconnect the earth from the mains lead to see if the buzzing stops. If it does you will need to get and fit an "earth / ground lift device" for a more permanent and safe fix.

NOTE: Disconnecting earth connections is extremely dangerous and is not recommended. The above information is for faulting purposes only and is not offered as a long term solution. Any concerns whatsoever with regards to electrical installations MUST be referred to a quialified electrician.

Other factors to consider include:

• Bad plug connections - Make sure that your connections are solid. Try wobbling the plugs to see if the buzzing is intermittent.
  
• Try different cables / leads. If the noise is only on one channel than it's likely to be a bad connection or faulty cable.
  
• Audio cables running alongside mains cables - Move them away from each other and keep audio and mains seperate.
  
• Mains outlet problem - Try using a different socket.
  
• Try a recording. The noise could be from an external source that you can hear on your PC speakers but not tranfer to the actual recorded material.
  
• Adjust the input volume. It might be set too high so it's picking up all the white noise in the signal and amplifying it.
  
• Get your equipment as close to the PC as possible. Running long lengths of cable between equipment can pick-up interference.