Whistling sound

We replaced the windshield on our '98 Z3 a month ago, and now, when we are on the open road above 70 or 75 mph, we get a whistling noise from
the lower edge of the windshield like someone blowing across a Coke bottle. At least we think it is the windshield, it comes from that area where the far edge of the dash meets the glass. It can come and go with speed, or it can go away when pasing a truck or going past a hill; only to return a few minutes later. It is very annoying, and I am wondering if the windshield is not set correctly.
Any Ideas?
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