Have you checked your coolant level? If the level is low, what you are hearing could be what coolant you have left boiling. Has your heater been working properly the past week or so? If coolant is low, the heater may not work properly. If the level is low, then you have to find out why, leaks, boiling off, never checked???
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.