Sounds like a blocked condensate pipe and the waste water has backed up into the heat exchanger so when the fan runs it bubbles and gurgles.. get a registered gas engineer to investigate and fit a new to the market device called a ‘boiler buoy’ if there’s any blockages in the condensate pipe DOWNSTREAM from the boiler so freezing or blocking is immediately noticible as the boiler bouy float will be visible. Yes a competant DIY’er can fit a boiler buoy without breaking the gas regs but there’s an equal chance that the internal to the boiler condensate syphon could very easily be blocked as well in which case you’ll need a gas safe engineer.
Phill Dee
March 2018