Ahhhh, Gwyn. I hate to be the messenger of doom but some years ago I had this exact same issue with an LT77 installed in my MGB V8 conversion. Changed all the bushes in the remote to no avail, tried fresh oil in the gearbox too, even some fancy additive. Still buzzy rattle in 3rd.
Ended up rebuilding the gearbox as one of the intermediate bearings had too much play. As I did everything myself it only cost me about £20 in parts but took hours and hours to get to.
I hope you can solve yours much more easily than I did
Sorry
