Thanks hmntp, some very interesting info.
I've googled some stuff about amplitude modulation which they are all supposed to generate when conditions are right, ie medium to high winds. Apparently there is a 'thump' roughly once every second which penetrates the structure of nearby houses as it is low frequency, a bit like the bass produced by a passing boy-racer in ASDA's car park - have you heard anything like that at R Grant's Bostonshire?