[pete-p]

Originally Posted by oldnick

I wonder if your idea about keeping your foot on the brake could be the reason

From what I've read elsewhere this could be the issue. The heat from the discs is causing debris from the pads to be absorbed/stuck to the disc - this is what creates the vibration and why it will come back again.

I guess it's not helped that our cars are automatic so there is a tendency to stop with the footbrake on, other than putting it in park or neutral (with handbrake on) there probably isn't much else to do.