My bike is driven 15000 kms and rocker box started noise problem since 8000 to 9000 kms. Even oil and oil filter is changed today but problem of noise is still going on. Can be rocker box adjusted? Or what can be done ?
Your Valve Lash Requires Adjustment
Valve lash is the distance between your valve stem and, in your case, the rocker arm. You have a camshaft inside your engine case or 'bottom end'. The reciprocating actuation is transferred to the top end via a push rod which contacts the rocker arm which actuates your valves.
Increased valve lash over time is a normal event and is a part of normal maintenance of your vehicle. Over time, as your engine wears in, and valve lash increases. The sound is a repetitive ticking noise that matches the RPMs of your engine. It can be a bit loud and annoying but it isn't a critical event. You do need to get it adjusted or do it yourself though, if not, it will continue to wear and possibly damage your engine.
Valve lash can be adjusted by removing the rocker box cover to reveal a threaded shaft and a lock not adjust shaft length.
Here is some additional information regarding your valve tick.
What you are most likely hearing is the Auto Decompressor which is used on the newer (2010+) Royal Enfield bike engines. It is used to automatically release the pressure in the bike for when you go to start it. Without it, there is the possibility of a kick-back on the engine when trying to start, which could cause damage to the engine/bike and/or the rider.
You can take it to a RE mechanic for adjustment, but other than the annoying noise, there shouldn't be an issue with what's going on.