Presume you have an analog speedo not the digital one.
In my experience it is the speedo cable from the gearbox to the back of the speedo binacle, one or two of the strands in the cable snap but stay in place, but as the cable goes round the free ends of the snapped cores catch on the inner of the cable casing making the clicking noise.
Sorry a tick in the cable as far as I know cannot be fixed other than by replacement. The tick is caused by the strands of the cable breaking, so its really a sign that the cable is starting to fail.
I've had speedo cable ticks on various cars over the years and they all needed new cables.