Wheel rubbing on brake block.

Hi,
During normal riding, sat down, the front wheel sounds fine. However if I stand up out of the saddle, I can hear the front wheel catching on the brake block.
Sometimes it's a repetitive noise every revolution (only one part of the wheel catching).
The brakes are direct mount so they can't have came loose and pivoted to one side. In fact there's a few mm clearance either side. I've tried reseating the wr skewer in the drop outs many times, including making sure the quick release skewer is done up tight. I've spun the wheel by hand, looked down and the rim looks to spin true against a line on the floor.
But !!! I've noticed I can easily push the rim sideways with a sideways press of my finger, right into the brake block.
Is this caused by spokes losing their tension ?
Thanks