Whilst in the cruise at FL240, the flight crew became aware of an unusual noise and an electrical burning smell. The noise quickly developed into a high pitched squeal, with some associated vibration and the smell became stronger, although there was no visible smoke.
After donning their oxygen masks the flight crew actioned the appropriate emergency checklist, after which the noise ceased. The aircraft landed safely at its planned destination. Investigation revealed the cause of the event to be worn bearings in the avionics blower fan. This is a known problem, and both the fan and aircraft manufacturers have taken safety actions to prevent similar incidents in future.