when you say it makes the noise as normal, does it keep doing this? as in runs on and on without firing until you turn it off?
if so, the problem is either a) the photocell is gone faulty (plug it out an let is run if it fires after approx 12secs, then goes out after 5 more, then replace the photocell)
b) if not it could be the solenoid coil (on top of the pump) usually this if the photcell is ok, can be tested for resistance if you have a DMM (should read approx 100 ohms)
c) finally if not one of the above, could possibly be the control box (replace to check)