Concorde
Not sure my report counts as the loudest, but in the late 1980s when the Poyle Intersection on M25 was being completed, I was tasked with the progress photography of the work by Cementation-Costain who had the contract. The time was circa 07.00 hrs. LHR ATC instructed me to hold clear of the climb-out path on the elevated slip-road section while Concorde departed. As that wonderful machine passed overhead 'twas the only time I had ever heard the sound of another aircraft while flying a helicopter.
More tittle-tattle on request! DRK.