Sounds like you had some schmutz on your connection. One of the symptoms will be that you will pick up more noise (including hum), and another symptom is that it will be sensitive to vibration (tapping on it makes it cut in and out).
The schmutz causes a high impedance connection, which still can couple in the signal, but the high impedance connection becomes more susceptible to pickup of stray magnetic fields. It's like you created an antenna.
The high impedance connection of the dirt also creates a variable resistor, which changes value under the pressure of vibration. The vibration causes changes in connection impedance and often causes a crackling noise as it changes value. Vibration of the surface the preamp is sitting on becomes modulated into the signal through this variable connection. It is one form of what is commonly called "microphonic noise".
Clean your connectors. I use rubbing alcohol.