Take out your valve cover and press every tappet with a screwdriver or something similar when the particular tappet(and valve is closed). If the tappet goes down and then rises slowly or isn't raising at all then you have it. No need to much force to check (about 10 kilos max) but you need to turn the engine by hand to check them all.
I hope that this will help you and I also watched the video and it does sound like a tappet and a really bad one to. You also maybe need to check if the tappets oil supply line is clogged...