Knocking noise, please help bit stumped

[Rob Graham]

I changed the gearbox, bought an LSD box from a lad in Washington n he said he got it rebuilt n never got fitted. I fitted box n shorter shaft n took for a spin n it has a knocking noise which speeds up n slows down according to wheel speed. The diff seems to have a lot of play but not sure as I've never really dealt with any LSD's. also when I dropped the gearbox oil back out it was glistening, has anyone had a gearbox rebuilt n the oil been the same, only drove 3 mile before taking back to garage. Just hoping gearbox isn't knackered. It's also had the housing repaired lol but the lad did seem genuine :S. Any ideas would be much appreciated