Mine did this some years ago, replaced the speedo cable, and it made no difference!
A few months later, while doing the heater matrix, I removed the clocks from the binnacle, and carefully lubricated the moving parts with some "air motor oil" the type used for lubricating air power tools, cured it instantly, and quiet as a mouse ever since...
Try this before you splash out on a new speedo cable.
Thom