G'day Renos,
might be backlash in the timing gears, but wouldn't expect it to be heard when the motor is running. When turning the motor over by hand you will hear a "tick tock" which is the backlash being taken up, caused by the valve springs/ pushrods working against the cam.
What about the timed breather cork? If this isn't the right thickness the breather could be rattling and the cam would have end float.
Just because they are new valve guides doesn't mean one can't come loose. When removing the old ones some metal might have been removed from the hole. That's why they make oversize guides in 0.001" to 0.004".
Motor noises can be a real bugga to track down.
Cheers
