I’ve always heard and believe (within an experienced reason) “A noisy valve is a happy valve”
Valve noise can be a sign of nothing to worry about OR that maintenance is due
OR that something is amiss.
Typically, with adjustable tappets, the valve system may exhibit some subdued clatter or whirring sounds. Singles and twins may sound like a clatter or "busy" sound, while the sounds of muli-cylinder engines are more likely to just exhibit a whirring sound because of the frequency of the valve opening & closing events. Think: music drummer, i.e. slow individual strikes are bang-bang-bang, but a drum roll sounds more like "RRRRRRRRRRRRRRRR". [can you do the common French RRRRRR with your tongue?]
As a rule, normal sounds from an adjustable tappet system will not be very prominent, because operational tolerances should be CLOSE to zero lash. What's zero lash mean? It means that there will be very small clearances that must be taken up as valves open & close. True "zero lash" is the virtually unmeasurable clearance between the valve stem and the rocker arm. A fully closed valve will have it's tappets (rocker arms/lifters/valve stems) relaxed so that the valve stem-to-rocker arm clearance will be either ZERO or a very small fraction of an inch. If that "clearance" was in fact zero (measurable) or an interference number, then there is a risk that the valve may not be fully closed (SEALED) when it should be. For that reason, on most pushrod + air cooled engines, we
usually set the valve lash between the valve stem and the rocker arm someplace between .001" and .012" depending on whether it's an intake valve or an exhaust valve. Those numbers are the typical specs over a broad range of brands and models. Intake valves usually run smaller than the exhaust side. The exhaust valves are often spec'd out to have more running clearance because they run hotter and, for that reason, we give them a bit more latitude in clearance so they cool well enough during valve overlap.
Typically, valve service procedures are done while the engine crankshaft positioning is placed so each cylinder in question is oriented at Top Dead Center Compression Stroke. I expect that BMW will follow those common methods.
As of this writing, I do not have specs on what BMW recommends for valve maintenance on the R18. Thanks to modern production and metallurgical knowledge, I expect that the specs will be pretty tight. I'm thinking: Intakes @ .001" to .002" and Exhausts @ .002" to .004", each at TDC-C of the particular cylinder. But I may be full of sh*t too! Time and more information will tell the tale.
If the R18 had hydraulic valve adjusters for EACH VALVE (like a common American V8) we wouldn't even be having this conversation. But hydraulic adjusters are HEAVY and having one for each valve adds complexity & costs AND reduces potential RPM. UGH! That's why most high performance engines (car and motorcycle!) still require regular valve system maintenance.
BACK to the first line of this post...
A well designed & maintained valve system that doesn't run at zero lash can be expected to have a very muted "busy" sound. If it sounds LOUD or as a prominent noise above other engine sounds, it should be checked out and/or compared to other (presumably healthy and well maintained) examples of the same design.
