What does a slow beep mean from a Klein non contact tester?
Have you read the user guide included with the tester? What is the model number?

I think Klein testers have a fast beep for higher voltages (48-1000 volts), and a slow beep for lower voltages (12-48 volts).

I'd use a multimeter (volt meter) to test the wires for voltage.  It could be that the non-contact tester is picking up phantom voltage, but you'll want to be sure before cutting the cable. 

If you have internet over the power lines, you could be picking that up.