Wiscos.....it's a bit more complex than a rubber doughnut which would probably tear itself apart over time. This is a photo of it in bits
The blue "grease" fills the void of the compensator and I believe takes on some of the load. Its not something that Morgan would talk much about...the workings of it that is so I hope they don't mind me posting a photo! . The last time I was at the factory they were about to use some alloy components in it to improve it.
Also behind your engine and infront of the compensator is the alternator which may be the source of your rattle.
