It is possible that what you heard may have been a rehearsal of this:
A statue of World War Two hero Air Chief Marshal Sir Keith Park will be unveiled on the Fourth Plinth in Trafalgar Square on Wednesday 4 November 2009.
Sir Keith commanded Number 11 Group of Fighter Command which had responsibility for the defence of London and the South East during the Battle of Britain and his inspirational leadership and tactical brilliance was central to Britain winning the battle, which in turn helped determine the outcome of the entire Second World War. His statue will be on the Fourth Plinth for six months and then a permanent memorial statue of Park will then be erected in Waterloo Place on the 70th anniversary of the Battle of Britain on 15 September 2010.
The Trafalgar Square event will commence at 4pm with (weather permitting) a Royal Air Force flypast of a Spitfire and Typhoon (Eurofighter, NOT Hawker!!) from RAF Coningsby in Lincolnshire, following which, supported by the Central Band of the RAF and the Queen’s Colour Squadron, the statue will be unveiled by Boris Johnson, Mayor of London, Terry Smith, Chairman of the Sir Keith Park Memorial Campaign and Air Chief Marshal Sir Stephen Dalton, Chief of the Air Staff.
4 pm is getting on for twilight, so may not happen.....