My 1999 986 Boxster 5 speed manual started having a slight grind when shifting from 4th to 5th. I am trying to figure out best procedure to repair this issue. Hoping it's not the synchro. I did an out change just before I noticed this issue. What procedures do you recommend to trouble shoot the problem?

If the gear box is going, do you know if the 5 speed transmission can be replaced with a 6 speed manual transmission?