Might be worth taking the cover off and making sure nothing is out of place mate. Noises like that can turn into bigger problems if you aren't lucky. Also need to ask have you had any work done recently?
are you sure the noise is coming from the rocker cover?
it may be that some of the lifters are now out of tolerance,
it could also be the spring in the gears that run the inlet cam, the inlet cam has a spring tensioned gear, maybe this spring has worn.
as for the gearbox issue, it sounds like the clutch and release could do with replacing. and check the clutch fork and make sure it is straight
are you sure the noise is coming from the rocker cover?
it may be that some of the lifters are now out of tolerance,
it could also be the spring in the gears that run the inlet cam, the inlet cam has a spring tensioned gear, maybe this spring has worn.
as for the gearbox issue, it sounds like the clutch and release could do with replacing. and check the clutch fork and make sure it is straight
See if you can make a video of the engine running for us
Injectors can also get noisy.