Eating dinner with my father at approx 2000hrs. a loud sound similar to that of a very low hot air balloon firing its burners came through the window. it seemed to increase and decrease in volume as though it was flying an elliptical course nearby, to the west of us. we do know that there is an raf airbase situated within operational range of our house and they do occasionally perform fly-over exercises near us. we are well used to the sound of these aircraft and also hot air balloons. the base has recently been assigned new aircraft, however i checked the sound of these new craft against a video i took of the sound, and, even accounting for distortion due to to topography, it is not the same vehicle. we listened to it for approx. 5 minutes before the sound went into the distance, to the south-west. throughout this time, there was no object in line of sight.