Is the problem on one channel, or with both channels, L & R??????
If the problem is in both channels, check out the output level of the sound source. If the source has a low output and the SP 505 has an INPUT gain too low, the sound will appear.
That sound always appears if your INPUT gain knob is at the max, and if the source has a low volume.
You must turn the output level of sound you're samplin' to the max without distortion, and turn the input level to the minimun.
If you touch the RCA inputs and the sound appear and dissapear from one or two channels, it's a problem of the inputs, maybe they have dirt or the plugs don't fit well.
IF the problem persist call 911.
Gear: Waldorf MicroQ, Roland VSynth, SP 808 and Juno 106, Korg Electribes ESX and EMX, Command stations XL7 and MP7, Yamaha EX5 and A4000, Ensoniq FIZMO