Rule of thumb, is normally, loose valves cause noise (as the tappet comes off the cam/rocker arm). However, I have had personal experience with tight valves causing noise (though, not on a Volvo engine). With that, I'll give you some maybe-not-so-necessary advice. Try to get your hands on a stethoscope. You will use it now, and forever, when attempting to diagnose a noise. When I'm hunting a noise, one thing that I observe is the pace of the noise. The faster pace (because camshafts turn faster than crankshafts) points in one direction, and vice versa. Since you're into the top-end of your engine, I'll assume you've heard what a loose valve sounds like. Is the noise similar? Usually, when my ears hear a top-end noise, I would describe it as a clank. When I hear a bottom-end failure, I would use the word clunk - there's more metal behind the noise.