2003 350z Manual

Okay so, I just rebuilt my engine but I'm having some transmission problems.
Those are:
- 1st gear and reverse popping/slipping out of gear when trying to roll out if given a rev of over 2.5kRPM
- Slight grind between gears.
- Rattling noise while car is on neutral, but it stops once clutch is pressed.
- While going up my driveway in reverse, you can hear a rattling noise coming from the transmission while reving the car. (it usually slips out when noise gets really loud)

All of this didn't happen before I swapped the engine. Trying my hardest to not have to rebuilt the transmission.

The attached video by the sounds of it, is the gears grinding, it stops once I put it in neutral and let go of clutch pedal.

 

The rattling noise is either the throw-out bearing (fairly normal) or the input shaft bearing (inside the trans case). Since you rebuilt the engine recently, I assume you installed a new flywheel, clutch and clutch components? Or did you re-use the old? You could have internal transmission problems. The 2003 MTs were notorious for synchro failures/issues.
-Icer