FE Hopey............Had some ice go thru number 2 once at rotation. It was so noisy in the tail that people had to yell at each other . No indication on the vib meter. Pulled the throttle back a little , noise went away , flew the trip and replaced 5 fan blades. I was amazed that it didnt show on the vibs. Was told that it senses sideways movement but not forward and back. There was no vibration in the flight deck and we would not known we had a problem if a flight attendant had not called us.