I've just acquired an HTC Vivid/Holiday/Raider hat vibrates every 15 seconds, nothing else.
The previous owner had a go at changing the digitizer and after which it did not work.
Looking over it, I have found the LCD cable has a tear in the corner.
The picture I have attached was taken at 20x with a USB microscope.
Due to the wire size, repair is extremely unlikely.
I have ordered a new LCD which I suspect will fix the problem.