ocasional sound from underhood passenger side
 

I have a 2002 Town and Country van and occasionally hear a sort of whistling sound from under the hood on the passenger side. I have heard the sound at different occasions both while idling at a stoplight and while moving. I have had problem with power steering hose failures on two separate occasions; the hoses were replaced and all seemed to be OK but the occasional sound has me concerned that there is a problem with the power steering. I have taken the van to the dealer on several occasions but of course the noise did not occur while they had it. I keep the power steering fluid in the safe level on the reservoir and watch it closely. Has anyone experienced this noise and if so what caused it? Thanks for any help you can give.