Ford Repair: top end noise, head gasket, noise question

Question
QUESTION: i have a 99 expedition with a 4.6 windsor.i did a head gasket replacment,the vehicle is running smoothly but i have a loud knocking noise in the valve cover area on the left bank.any ideas as to what it could be?

ANSWER: faulty lifters would be my guess,, did you replace them when you did the head or did you use the old lifters ??

---------- FOLLOW-UP ----------

QUESTION: i did not take the head apart,i removed it replaced the gasket and put them back on

Answer
why did you replace the gasket ?? oil leak ???? the other possibility is that the timing chain tensioner isn't tightening up all the way. I know this sounds weird but, drain 1 quart of oil from the engine and replace it with 1 quart of marvel mystery oil and take it for a trip up the highway and recheck