I have a knocking noise that goes away after awhile but is still loud any help?

What is your question?

Ok so I just got the truck and it's still under warranty, but when I start up the truck. It starts to tick or knock pretty loudly until it warms up and when I accelerate before it's warmed up, it gets louder. Also my oil pressure gauge fluctuates when I accelerate and is below forty psi now. I have heard to add Lucas oil stabilizer and that it might just be oil dropping to the pan after sitting for a while but I would really like some advice because I'm not all that experienced with vehicles.