The Airbus A320 series of aircraft are the most common type we see overhead, they are operated by EasyJet and British Airways among others. These aircraft have had an issue with a high pitched noise caused by the fuel overpressure protection (FOPP) vent on the underside of the wings. This problem can be addressed by retrofitting an airflow diffusing device, shown in the middle image (left).
EasyJet and BA have been persuaded by our MP, Greg Clark, and further encouraged by the Arrival's Review Team's recommendations substantially to complete this at Gatwick by the middle of 2016. This work is not yet completed.
Lufthansa (107 million passengers in 2015) and Air France (79 million) modified their fleets by early 2015, easyJet (70 million) may be half way now, British Airways are not saying.