3rd gear, strange sound
 

I recently purchased a used transmission for my 73 2.0 914. Got it on the car last night and it behaves a little strange. R,1,2,4,5 are all very easy to get into...a little too easy, 3rd however doesn't like being touched. It sounds like the gears aren't really catching in 3rd, grinding. Lots of noise. Is this a clutch problem? Should I pull the tranny back out of the car and crack the case to have a look inside? Not sure I'd really know what to be looking for anyway. Any Suggestions?