Leyland Daf LF 45 - 2003, loud noise when accelerator pedal pressed?

Hi
My horse lorry usually runs like a dream however on the weekend while cruising on the motorway a loud siren (almost drill noise) came on while I was pressing the accelerator, if the accelerator was 'pumped' 3 times the noise would go, the lorry lost no power and the temperature remained the same, when the noise was happening you could almost hear an airy sound but again my air tanks remained full. Any suggestions?