First thing to do is check the oil level. If the noise is a crunch, then you're beating the synchromesh. Try a slower change. Also try the double declutch test:
Clutch down - into first - clutch up - move off
Clutch down - out of first into neutral - clutch up - clutch down - into second - clutch up. The clutch up/clutch down between gears helps to synchronise the hubs, and with practice can be done quickly. If the problem is a crunch, and this method gets rid of it - it's the synchro on second gear.
It might also be that the clutch is not disengaging properly, but I would expect a bigger noise changing from 3rd to 2nd with either cause.
