Fuel system?? 96 Blazer check engine light

My wife had the car for a bit this evening. The check engine light came on. She drove it home without problem.

When you turn the key to the on position there is usually a whirring noise. This is the fuel pump priming right??

Well when I turned it to the on position it still makes the whirring sound but there is also a clicking sound for a few seconds. No problems starting it. When it runs the whirring sound almost sounds like it is kind of making a pulsating volume difference. The check engine light stays on. I had my fuel pump replaced about one month ago. T

the sounds come from the driver side under neath the vehicle kind of towards the back tire.

Any ideas of what this might be??

Thank you in advance for your time.