On the JTD 16v/20v waterpump it's the bearings that go, causing leaking coolant and possible seizure, resulting in cambelt breakage (see the sticky thread on this topic). If it is the waterpump making this noise then the bearings could be on the way out. Either way you're getting the cambelt/waterpump changed anyway so this could solve the problem.
Do make sure the tensioner and idle bearing are also changed. The tensioner is on the schedule for change but the idler isn't. This can also seize,causing cambelt carnage. The JTD 16v cambelt/tensioner/idle bearing come as a kit from Alfa Romeo, so there's no excuse not to change it.