Could it be an L188?
Well chaps if you have not noticed since the 1st of January this year an Atlantic Airlines Electra night stops at Bournemouth each night.
Monday to Friday morning this aircraft departs at approx 0450 local time down to Jersey with there mail and papers.
On Saturday and Sundays I think it is both days the aircraft goes to Guernsey a little bit later so was this the aircraft that was making the said noise on the day inquestion?