Hello!
I started another thread about headgasket or waterpump leak.
But i want you superduke owners to watch and listen on this youtube clip.


https://youtu.be/LkAhMsZILfQ

The engine is just started and run untill the cooling fan started, then shut off.
Do you have this sound on your bike?

Im worried that something big is wrong.

Best regards/ Fredrik

Strange one this - putting your two threads together, sludge in coolant is definitely one symptom of failed water pump seal - maybe the 'boiling' noise is due to parially blocked coolant passage venting into air pocket.

Suggest start with pump and flush out whole system thoroughly with hosepipe, both ways through inlet and outlet - not likely given age of bike but you might still have casting sand in there.