Doepfer

heres a question about the doepfer bbd module function:
 
even without an incoming signal and with feedback set to zero, as i sweep clock time from minimum to maximum, i hear an audible oscillator that increased in pitch until around 12:00 position when im assuming its going above audio range.  i cant help but think this is a defect.  is something wrong or is this the way it was designed?  if its abnormal, any suggestions as to how to fix it?  
 
thanks
mini

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