maybe the noise did come with age of the engine. i do some reading on the manual, it was saying the noise is due to defective hydraulic lash adjuster. after some thinking i'm thinking it might be correct, i'm not very sure what is the actual function of hydraulic lash adjuster. but refer to the position of the adjuster seems like it is the parts to maintain clearance between rocker arm and exh/suc valve. if it had lost its tension and it will create clearance between arm and guide than it will have the noise whenever engine is revving. but as nicely mention it might due to Spring tensions?? which i think it is the same concept. anyway if really done not much harm than forget it!! bring up the volume of the music than.