I flew a 1-11 475 freighter which was on contract to DHL flying parcels at night - approx. 0330 local take off - from Brussels to Stockholm. Usually departures were from 25R with a climbing right turn to head off on as straight a track as possible to Sweden. Anything up to 40 aircraft would depart in the period 0300 - 0500 but most were turboprops and our two RR Speys were the noisiest of all. The good burghers of Brussels had an early wakeup call and were not amused. However one night for some reason there was a delay in departures from the 25 holding point and our crew, as they left the ramp requested 07 for departure which was approved. A couple of days later it came to light that a special portable noise monitoring device had been positioned on our normal departure route to catch us out and purely by chance we had foiled their evil plan. We heard no more noise complaints!