Bizdev
Many thanks-I do seem to recall this was transient event but a very noticeable on the Tridents,I don't seem to recall other planes doing similar. I suppose it is one of those nice quirks of the time that came from having so many different types of aircraft. Other favourite oddities of the time were the great noise Swissair CV 990 s made and the enormous moving silencer contraption on the back of Alitalia DC8-40 engines . Apologies for thread drift into general nostalgia for LHR in the late 60s Thanks again
PB