Astra 2004 possible EGR fault?

I Have a 54 plate Astra 1.6i /twin-port. The problem i seem to be having relates to high CO emission readings of 3.20 compared to the Normal 0.2. but the engine seems to be sucking in Air. Each time this happens i get a low squeeling noise . If i release the Oil filler cap there is a release off pressure. The squealing noise stops. Could this be down to the EGR valve