My car at times makes a rattle / chattering noise when the car is under load, if I put it in neutral and rev it, the noise is not there, so it has to be part of the drive train. My transmission shifts rough, but I spoke with the transmission company that did a shift kit and valve body rebuild on the car before I bought it... Could it be a cracked or broken flex plate? How would I check without pulling the transmission out?

 

sounds like the exhaust hairline crack a lot of people had a long time ago. Check under your car towards the engine where the y pipe is and look at the brackets and see if there is a crack. Common issue. I am pretty sure there is a post on here for the fix it.

 

I'm having same issue with my maxima especially when hitting 5K+ RPM while in gear. Guess i have to replace mine, last time i checked it was cracked.