Large rectangle with red and green flashing lights and slight haze, created very loud bass drone followed by quieter drone

Large flat rectangle with red, green and white flashing lights and slight haze, approximate in proportion to a smartphone. Created a very loud thunderous drone which was heard first prompting me to look out the window, very loud and drowned out music that was being listened to. Was flying very close overhead then dipped forward in a steep curve towards the building opposite, at which point it was no longer visible. It seemed only one side (the base) of the object was visible. A different quieter mid-frequency drone lasted approximately 15 minutes afterwards. Sound of close proximity helicopters was also audible for a considerable time afterwards but not visible. Location is very close proximity to an arms and aerospace factory.