well start by weeding possible causes off the list, like the starter for instance. Not the problem. Have you checked the Battery voltage? should be about 12.2-14 not on, 13.6-14.5 while running. Stupid question, did you check the oil? if they forgot to put oil in it or forgot to install the plug, then that would explain the loud engine problems. Have you checked the trans fluid? brown=change, black= change/replace filter, red=good to go. Have you checked the engine mounts for damage? that can add to the violent engaging. You could have bad Camshaft Position Sensors. They cause what you are describing exactly. If your trans fluid is red, then your transmission should be fine. I would start with checking the fluid, then the mounts, then look at replacing the Camshaft Position Sensors. If you replace them, it takes less than 30 minutes to do. Make sure you get Nissan Replacements. Other people have had problems with Aftermarket sensors. Bad CSP sensors cause erratic shifting of the transmission and starting issues. Good luck and keep us posted.