[DonSor]

I don't know if it applies to your year truck but my OBD II states that although a steady lit Check Engine Light (CEL) indicates a problem, it is not critical in that you may continue driving until the problem can be taken care of. However, a flashing CEL indicates a more serious problem and for the operator to safely but quickly come to a stop.

If you have an ignition problem you are going to know it. When I had the noise (more like loud humming during idle) similar to what you described, I replaced the Idle Control Valve and it corrected it.