So I have an 2002 zx2 with an auto trans. It makes a nasty knock when I start it, but after it starts it sounds fine. The car has 69,300 miles. My college age son purchased it a year ago from a used car lot. It has been sitting mostly unused for over 9-10 months. When he was last driving it regularly, it developed a leak in one of the transmission lines. He noticed the car was having trouble shifting while driving to his first day of class at the college he was attending. He drove it low on trans fluid over 60 miles to and from school. Surprisingly, it made it home. So, I changed the rusty lines and filled the transmission to a correct level, and it seems to shift correctly. The issue is as stated, it still makes the knock at start up. Last fall I checked all the easy stuff. There were no loose engine mounts, exhaust parts, heat shields ect. I also pulled the serpentine belt and checked the alternator, p|s and a/c. They all spin free. I checked the starter also, it seems fine. The knock sounds internal, but it's not faint by any means. My best guess now is that the flex plate is cracked. I really don't want to put in all the work tearing it apart, to find out that the flex plate is fine. He was trying to sell it, but no one was really interested. He's looking to move out, so I gave him some cash, and now this car is my problem. So, does anyone have any ideas?