Basically I have fitted myself a AFR gauge a few months ago. I have always noticed a humming noise when driving along motorways etc. But I have always put this down to tyres or gear box. I have noticed for example when my car is at idle it does not just stay at 14.7 but I continuously fluctuates between 14.7-15.1 and back. Now it also does this when I am cruising along the roads at a continuous speed giving a two tone noise from the engine that I thought was the gear box at first. Now I have a standard fuel pump in the car, 111k on the clocks now, and running a 4bar reg. could this issue with the AFR going up and down slightly be due to the fuel pump being tired? any other suggestions to the possible cause?