okay so my engine started knocking a few days ago. It goes on through all rpm's but does quiet some at the higher rpm range and when the car has reached operating temp. The knock is a constant rhythm also. Oil pressure is around 40 to 50 when the car is cold but drops to 20 or 30 when the engine reaches temp. Please help i have no idea where to start with this one!

constant rhythm knock? does it speed up with rpm speed?
if it sounds like this
http://www.youtube.com/watch?v=WA4G_LDEflo
then your engine is toast