Hmm. Not sure what to say about the oil leaking. And I wouldn't think it's the timing chains because the tensioners have a ratchet that keeps them from loosening. But the loud noise could easily be the lifters not fully pumped back up. When I put a new motor in my car back in 2011, I had the same thing happen. Since it was a junkyard motor, it had been sitting for at least several months. When I first started it up, it made an awful racket in the top end for about 20 seconds, then went away and has never happened again.
Here's something to try. The next time you try to crank it after it's been sitting for a while, first: mash the gas pedal all the way to the floor (and hold it there). Then try and crank it for a good 10-20 seconds. This will allow the motor to crank, but not start. If dry lifters are the issue, this should prime the oil back into the head and pump them up properly. Then start it the regular way and see if it still makes a racket.