Strange noise after start-up
 

Hi folks, for the past couple month I have noticed a noise after start-up that I haven't herd before. It sounds like my fuel pump (Raptor 100) is cycling like a two speed pump. The truck starts fine and the noise starts up after about 30 seconds, then it cycles a hi and low pitch sound every 30 seconds or so for a couple minutes then is becomes steady like a fuel pump. I hear a relay switching as it cycles but I can't locate it. I can temporarly disconnected the power to the fuel pump and the noise goes away. I have changed the fuel filters and checked the Raptor fuel pump inlet screen, no change. The truck starts and runs fine, I just don't want something to fail and leave me stranded. Is this normal, maybe the alternator is re-charging the batt's after starting and the voltage is fluctuating causing the variation in noise.
Any ideas or suggestions would be appriciated.