Thanks for the replies. It definitely was'nt a stereo system Ian, as the pauses between the "thumps" were too long and it went on for about an hour so I reckon it must have been the windfarm site. Coincidentally, there is an exhibition in the the Venue Cymru next Monday between 1100-1700 about the proposed plans for the new, bigger windfarm proposed for the area between Ynys Mon and the Isle of Man, so I might ask them about the noise if I go.