98 Taurus engine knock

I have a 98 taurus with the dohc duratec engine 145k. while returning home from a 600 mile trip the engine started knocking, it seems to be coming from the cams on the side closest to the firewall. It doesnt seem to be affecting the engine at all it still runs as usual, its not using or losing any oil. I have tried changin out the 5W oil and puttin in 10W and that did nothing, tried to use engine restorer, that also did nothing. Is this a serious problem and is it ok to continue driving it with the knock. Im going to be replacing the car this summer and dont want to put more money into it. Any help would be GREATLY appreciated...thanks for reading!

Does the frequency of the noise change with engine speed? Is it definitely a metallic sound?

Does the tapping noise seem to be coming more from the top of the engine (I realize you mention the area of the cams) or from the bottom? That close to the firewall makes it tricky as sound could bounce around back there.

-Rod

Yes the noise is metallic and the frequency does increase with engine speed, the higher the rpm the faster and louder the noise gets, and it is coming from the top of the engine. i noticed today it doesnt seem to get loud until the engine has warmed up, when i first start the car when cold the knocking isnt there at all but once i actually drive and accelerate it comes back and then continues to knock when idling