Noise in 3rd gear

[Chipper]

I recently changed my worn-out standard Minor ribbed-case gearbox for a used MG Midget close-ratio one to match the 1275 Midget engine in a 1970 Traveller. It was purchased fairly cheaply, and I opened up the side hatch prior to fitting it to take a look inside and all appeared pretty good.

Having installed it, all is well, apart from a whining noise in 3rd gear and 'chattering' of the gear lever. There don't appear to be any synchromesh issues (unlike with the 'box I swapped it for). Are there any known symptoms or cures for this? I was thinking along the lines of Lucas transmission fix or similar...