cook07

i have a 07 f150 with just under 90000 miles on a 5.4 triton. ive had the truck for almost 2 years and i recently bought a 27' pontoon boat. after pulling the boat about 180 miles or so back to my house when i stopped put the truck in park the motor was rattling and knocking so bad it looked like it was going to jump out from under the hood. i can accelerate just barely while in park and it stops... same if it were in drive... as soon as i hit the gas it clears up. the only time it does this is when i have been pulling this boat and i come to a stop. it runs great as long as im moving with no noise knocking or anything. it also runs fine with no load. i hope somebody can help... and thanks for the advise in advance