Engine makes a high pitched whining sound while running,

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We thought it was a leaking vaccuum line but they are all secure. it seems to be located around the left bank or odd number side around no.3&5 cylinders,maybe under the plentium. it It seems to decrease after the engine has run for about 45 miniutes.When I turn the engine off it fades away. I have had 3 machanices look for it including myself we can't find were it's comming from..