1998 5.2l p308 cylinder 8 misfire

[idrthrbfishing]

I have a 1998 grand with a 5.2l engine. last week I posted on a new noise after a large puddle got in my way. (it won) I got a mech to check the codes. there were just a few 16 to be exact--- needless to say could we not pin point a thing. after about 3 miles the light came back on. the code states a p308 cylinder 8 misfire. to me it sounds like its trying to fire and the exhaust valve is open giving it a loud tapping noise.this noise seems to either be there when I start the engine or not...if its tapping it does not stop/ if not it does not start. when I pull the plug wire it will stop tapping. now my mechanic wants me to put some engine flush and valve cleaner in the engine and then change the oil. has anyone had any issues with this or heard of this issue?
I don't want to swallow a valve or do any more damage if I can avoid it.
thanks.