Hi this may be a bit late, but I have only just joined. However, The speedo alarm as fitted to Sauddi spec motors is silenced by diconnecting a wire. The wire is in the loom that comes out of the back of the Fuse box under the dash. The wire you need is a yellow wire that has an in line spade...

Have you looked at bushes, especially panhard rod and what about your swivels. Also vibration at higher speeds can be prop UJ especially front ones. Hope this helps. Stuart