Does sound like a characteristic CV noise mate - they tend to make loud clicking/clonking/banging sounds when under load if they're shot or not fitted properly. Double check all the driveshaft fittings and CV joints closely....easy to pop one of the bearings out of the CV joint under the gaiter without realising. If needs be, strip out driveshafts and have a good look at them, possibly think about re-ferb'ing them while you've got them out? Replace gaiters, rinse out old oil and grease using a good kero bath or loads of WD40, then re-pack grease and repace gaiters. If you've got them stripped out, always worth the hour or so's work to repack everything. Make sure the driveshafts are fitted into the gearbox housing properly - a plastic/copper hammer and a good clumping towards the gearbox, just to make sure they're seated fully. Been known for a driveshaft to look fitted properly, but not quite past the sealing collar into the gearbox. Once re-fitted, ensure gearbox oil level's about right, and re-test. If the CV's have gone, it'll be relatively obvious. If it's the new gearbox - and the oil's at the right level and been allowed to run up to temperature, chances are it'll be the new gearbox with a problem. Might be teaching you to suck eggs chief, but does sound like a CV problem. Be prepared for the worst though! Hope this helped some...