Hard tapping noise coming from passenger side of engine

I have a sohc. It was making noises and I knew it was a timing chain cassettes. I put some Seafoam in it and about three days later a knocking started coming from the passenger side. Put it on the computer at AutoZone they said that number two cylinder was not working. I change the oil and put a quarter cup of STP in it and the noise seems to be a lifter or a broken valve. I was wondering has anybody ever experienced this and could it be caused by the timing chain?