Top end rattle/tapping after valve cover gasket replaced

[V8 Ben]

Hi everyone,
Replaced valve cover gaskets on my 4.6 yesterday which took all day. Couldnt seperate the injector plugs from the injectors so had to lift the entire fuel rail and pop the injectors out.
Upon removing the injectors, i was greeted with the sight of hundreds of little shells the size of acorns of which a few fell into the injector holes as the injectors were lifted out.
After wrestling to get the passenger bank cover back on (I have a RHD car), everything was put back together.
I removed the spark plugs and cranked the engine just incase one of the shells i mentioned got sucked into the cylinder - that way it could be ejected from the compression chamber without causing damage.
Plugs back in and it was idling fine, but with a loud tapping coming from the passenger bank (the same sound you get before the oil reaches the top end).
This sound is now constant.

Any ideas on what could be the cause?

Thanks in advance

-Ben