valves ate the ones responsible for allowing fule/air go itno the combustion chamber(intake valves) and expelling the used gas/unburned fuel(exhaust valves) and the lifters are the ones that pushes the valves to open and cole along w/ the rocker arms....on the ticking side you may want to re-torque the bolts that secure the rocker arms(mine ticks too but i need a new head cylinder now)....
ticking that ceases at higher rpms is almost always the lifters. they wear out and the tolerances get slack, so at the "dead level" in the cam (when it's not pushing at all) there is a gap between the cam and the lifter and you get that metallic click. The reason why it goes away at high rpms is because the lifter spring doesn't rebound quickly enough to reach the equilibrium point.
I've never done valve work before, so I may be off a little. I'm replacing the lifters in my 3000GT next weekend, so by then I'll be a pro, but from what I've seen in the 1.8L BP when I broke it down to change the valve cover gasket, it did not appear to have any rocker arms. I think that rocker arms are only used in pushrod engines where the camshafts are below the crankshaft. In the BP, the cam lobes directly contact the lifters, which sit right on top of the valves. I think that is right, but somebody correct me if I'm wrong.
ok heres were i can be used. first off the 1.8 uses what are called tappets not liffters, 1.9 uses roller lifters the lifters roll on the cam shaft and push directly on the rocker arms wich push down on the valves wich let air and fuel into the cylinder and exhaust escape. and i do not recomend you to do your own lifters if you know nothing about engines, or at least if you dont want to risk messing any thing up. the lifters have to be facing a certain way and should not be swaped around. the valves in the 1.9 are not adjustable. and the valve cover bolts have a reputaion for breaking when taking them out or putting them in due to they are made of such a soft metal. a tick does not always mean lifters, let some one listen to it. my wifes scort had a tick and it ended up being the cam gear was broke in the middle.