IF and when you remove the speedometer you will find a metal cover on the back of it that can be removed by taking out a few screws. You will then be able to see the internals of your speedometer.

Where the cable goes in there is a bearing which holds the spinning magnet in place. This is not complicated and when you can see it you will see how simple this is.

That bearing will seize if not lubed every ten or fifteen years. You say you had the speedo serviced about two years ago. I doubt the shop replaced this bearing as that is a massive job for such a small bearing.

Lubing the cable is always a good thing, but if the noise persist then you might consider removing the speedo, pulling the cover and lubing the bearing. I usually use "3 in 1 oil" with the blue cap.

There are also a number of other pivot points in there that could do with a bit of lube. A tiny bit is all that is necessary for many years of service.