Zetec Se Question

Hey all just a question realy when driving along the road or up hill around 2.7k rpm and if you hold the revs there. The engine makes a realy faint noise like you can hear the piston moving up and down but it's so faint and I do know after 3k rpm the engine note does change. Im sure it isn't bottom end as it has had an easy life and services every 5 to 10 k always topped up so never been low on oil. Uses anything up to 100 ml every 200 miles depends how it feels each month. There is no noise on idle except for clutch release bearing rattling. Could it be that whilst I'm moving as there no noise after 3000rpm either just between 2.5 and 3. I have had it sat at around 3krpm and used a steps scope and listened around the engine and I do have s noisy tappet and 3rd injector just wonderd if it's normal as never heard it before but not like I been listening out for it either sorry for the essay tried to give as much as I can

Thanks in advance for all your help and time to read