Very loud noise from roof of E350 passenger van

I've searched all over the net and newsgroups and can't seem to find anyone that has experienced the problem I'm having. Our E350 15 passenger van has just recently started making a very loud vibrating
noise that comes from the roof. When it started, you'd have to be doing 70 mph to make it happen. Now it happens at 50 mph. The closest thing I've found online is someone talking about a harmonic vibration on an excursion they had. Could that be it? If so, what could I do about it.
Oh, it's probably worth the mention that the previous owner drilled a hole near the front center of the roof for a CB antenna. I removed the antenna base (the antenna itself had been removed long ago) and plugged it with some stuff that's like JB Weld, only thicker. It dries as hard as steel. We thought that doing that fixed the problem, but the noise came back in 2 weeks.
If anyone has ANY ideas at all, please reply. We use this van to transport children to and from school for our daycare. The noise scares the children.
Many thanks for your time. Max.
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It's probable that one of the cross braces is bent and/or not making proper contact with the sheet metal on the roof. Dodge vans were notorious for the cross braces actually buckling from the wind battering and body flexing after a few years.
Fords usually fare much better, but if the previous owner had a cb antenna on it, all bets are off. He may have drilled through a brace, or simply the weight caused by the wind load on the antenna may have damaged the roof.
The only way I know of to fix it is to remove the headliner and see where the problem lies. You may simply be able to find a gap between the roof and a brace and wedge something in place to keep it from flexing.
CJB

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That makes perfect sense. I've never dug in to body work, so it didn't dawn on me that there would have to be cross members up there. It's possible that the previous owner stood on the roof to put the antenna on. Plus, they used it to haul 10 hunters at a time down to Mexico. It could be that they strapped "who knows what" to the roof during those trips.
Thanks for the info. I'll pull the head liner down today and see what it looks like in there.
Max.
says...

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Maybe the antenna lead is still there ?
Bill
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I would take a large magnet and place it on the roof over one of the braces. If the sounds was still there, I'd move it over another brace until I found which brace was loose. Then you know which part of the headliner to remove to fix it.
Large magnets can be bought very cheap at Harbor Freight tools online. Hank
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We found the problem. One of the things that made this hard to pinpoint was the fact that it would never make the noise while I was in the van. It only happened when our teachers were driving. No offense to them, but none of them are mechanically inclined. Today was a nice, hot day here in Houston Texas (85 deg F) and very windy. So I took it out for a drive and heard it for the first time.
It turns out the rubber molding that covers the gap between the windshield and the roof had come loose in the middle. When the wind hit it just right it would flap very quickly... and VERY loudly. A little cleaning and some silicone adhesive cured the problem right up.
Thanks to everyone for the ideas. I had actually pulled the head liner down and found no problems with the crossmembers. It was just luck that I noticed that molding.
Max.
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