I'm hearing a loud knock in the morning when I start the engine

 

Vehicle details such as MY, engine size would be very helpful.

 

Seconded on details. A common issue with the 5.0 on these is the timing chain tensioners and guides seem to wear out quite quickly, but that's more of a RATTLE. If you're getting a KNOCK on startup, check your oil level; does it happen if you rev the engine in neutral/park?