Mystery noise coming from Van wheel area.

Pull the brake drum completely off.

It could be a tiny iron stone pebble that is probably as hard as the brake drum. It is going to be impossible to find without disassembly. If your mechanic didn't pull the drum off and just checked the bearing by jacking and wobbling, then if it's as I suggested it would not have been found.

I had this exact iron stone pebble once and the noise was horrifying

Scratcher.
Did the mechanic remove the hub/drum off the axle to check bearings? If NOT, he hasn't checked the bearings at all, just felt the adjustment slack if any!
If the hub was removed from the axle then any anomaly should be able to be identified. ie, stone as above.
It is normal for an electric brake magnet to hang on the drum and flutter sometimes. A slight mount of braking voltage, ie, minimum controller setting, should adhere the magnet to the inner drum face but not apply the brake during forward motion and stop the rattle, but some slight scraping may still be heard.
Without removing the hub, but having the brakes backed off so NO shoe contact is happening is the only way to hear the magnet. rather than the shoes scraping slightly. Adjust properly afterwards.
If hub hasn't been removed you cannot discount any magnet problem even though they may apply the brakes as normal. They DO WEAR THINNER, and OR tapered or BOTH and should be checked for thickness and integrity.
Any decent real check should ID the problem IF abnormal in sounds!

Another possible is, Some hub/drums have insufficient stop facing inside the drum to accommodate the head of the wheel studs to be below the inner face where the magnet runs. The stud heads, only takes 1, if becoming proud of the surface will flick the magnet as the drum rotates. Again, it will be seen if apart and LOOKED AT.

thanks for the help Guys,
but as I speak my local mechanic has just found some slack in the bolt mountings at both top and bottom of the shockers. Not sure on a solution just yet. Will keep in touch.
Scratcher

Hi Scatcher, I had the same issue on our big lap and fixed it in Gympie in Qld. the problem was a magnet had come away from its mounting, and had been doing it for some time. I had to replace the whole brake assembly and backing plate . I still to this day still don't understand how it came off the hook on the magnet arm. I hope this helps. good luck