Outer Limits 4x4 Board

Hi all, I have a 99 GU patrol 4.5ltr petrol/gas.Im chasing down this noise I have ,when I switch off the engine I get this whistle that fades away and also this woowing noise when the car is running. I have found when I pull out the oil dip stick slightly or undo the oil cap filler and relieve some pressure, these noises dont happen. could this be high oil pressure? I dont get any lights or anything. Im struggling to pin point the where abouts of this noise too, but its the engine area. These noises also only happen when the car is FULLY warmed up, does not do it when the car is cold or just a little warm.
Could it be the relieve valve on the pump?
any ideas?
Cheers
Glenn

maybe is just the crankcase pressure which is normal and the whistle maybe coming from your breather which normally comes off the rocker cover maybe a possibility but if worried about all pressure put a mehcnaical gauge on it and check