Hello, I hope the following is of assistance.
The valve clearance is adjusted hydraulically so no you can't adjust the valve clearances the adjustment is automatic.
With regard to the noise you need to confirm that it isn't a light diesel knock as it's quite common on this engine for a failing injector to cause a knock. The pilot injection and fuel pump values can also be relearned, plus the PCM calibration will need checking to make sure it's the latest level available.
Unfortunately to relearn any values and check the software requires a suitable diagnostic tool such as Ford IDS so it isn't really a DIY job.
One other cause of a rattle although this tends to be more at start up is the timing chain tensioner failing, the tensioner has a mechanical ratchet to hold tension on the chain when the engine isn't running, once the engine starts the hydraulic action of the tensioner works.
I hope this helps but if I can assist further or if you have more questions regarding this let me know and I'll reply as soon as I'm able to.
Hi, the noise is only evident on high revs or pushing it up hill, if it is worn rockers would changing the rocker frame be any good?
if the rockers are worn I would expect you could see flat spots on the roller that contacts the camshafts.
Pushing the engine would still suggest that it may be a diesel knock, but obviously without hearing the noise it makes it quite hard to diagnose.
If you replace the frame the rockers come with it so if the noise is this then replacing the frame would cure the concern.
Let me know if I can assist further.