Weird Buzzing Noise from 98 Windstar

I have a 1998 Windstar (3.8 litre) which has started producing a strange buzzing noise. It sounds like a vu-vu-zela or someone blowing over the top of a pop bottle. The noise is intermittent - sometimes it doesn't happen for several days. The noise is not continuous; it lasts for a fraction of a second to about two seconds at a time. You never know when it will start; then it may occur every few seconds for a few minutes, or only once the entire trip. The volume can vary from barely being able to hear it, to loud enough to startle you. It seems to be coming from the rear, but it's difficult to say exactly where. Once I thought I head it when the van was stopped, (with engine running).
Hope someone can help with this one.
Thanks.