1999 GMC Sierra 1500 c/k two questions I have

Every once in awhile (ussually when I drive long distances, and sometimes when it is cold) my truck makes a loud ticking noise. It doesnt go away while im driving, it sounds like a lawn mower to me. Would like to know if you guys know where the ticking is coming from. Thank you. I have one more question also, I have been leaking transmission fluid what should I do to fix it thank you