Noise from engine

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has anyone experienced a noise similar to air pressure relief valve going off when vehicle is under load / accelerating ?. I have a 2002 3.4 litre V6 Prado have 105,000km on clock . Our mechanic thought it may be a faulty petrol cap but have changed it out with now difference, He has also discounted a faulty oxygen sensor it is spasmotic and only last for a second. Any suggestions would be appreciated