Jeep knocks on startup, then goes away.
This is going to be a very long post.....sorry.....I have a 2004 Jeep Grand Cherokee Freedom edition (regular laredo with imo nice appearance package.) 4.0 auto trans. I have owned the Jeep for about a year. It currently has 93k on it, had 85k when I bought it. have changed oil roughly every 3000-3500 miles. Recently changed the oil with the usual 10w30 castrol gtx, put in the regular 6 quarts, but used a mopar filter (figured as with my toyotas the factory filter must be good). Everything was fine, drove to work and home, went to crank the next day, and heard a really bad knock (sounds like a rod knock).oil pressure slightly above middle.. turned the jeep off immediately. when starting the jeep again, it cranked for about 2-3 seconds then started, very slight tapping sound, but nowhere near what i heard at first (oil pressure still good). Drove the jeep and tapping slightly dissapears. Figured I had got a bad filter. Got Napa gold filter changed with 5 quarts of valvoline high mileage, and added a quart of Lucas HD oil stabilizer. cranked engine, recieved exact same knock, shut off immediately, cranked again, engine noise slightly there. Drove for about 15 mins, engine noise gone, runs great and great oil pressure......Figured I would go back to the normal oil filter, fram tough guard TG16. Still having exact problem. Jeep runs great after second start, but still knocks loud at startup until i shut off and crank again. when I crank again still takes 2-3 maybe 4 seconds to start, but when it does the very slight tapping sound almost dissapears. This sound is always towards the rear of the motor closest to the firewall. There is no noticeable vibrations in the shifter or on the motor when the hard knock is present. I am very stumped...going to check flexplate bolts, but this is nowhere close to what i have read on them. Worried i have a broken piston skirt or bad rod bearings......Also, dont think this has anything to do with it, but I do have a very very small rear main seal leak. If anyone has any ideas please let me know....very close to taking to stealership, or doing rings, rod bearings myself. This shouldnt be happening on a 93k engine. Sorry for the insanely long post, but want to give all info I can, hoping you guys can help. Thank you for reading.