briggy wrote: would a loose bearing housing give a loud tapping noise?
Yes.
As has been suggested, the noise you hear might be normal, but if it's loud enough to be alarming then it could be contributing.
It's not the best engineered detail in the cylinder head, relying on a gasket to do double duty - seal oil leaks, as well as press against the bearing retaining sleeve (or bush) to hold it in place.
A good suggestion I read here is to give priority to firmly locking the bearing sleeve, and use supplementary gasket paste to ensure oil leak control.
Maybe even delete the gasket altogether.
A possible complication is that the bearing sleeve has been loose long enough to make the hole for it in the head over-sized.
A fix might then be to use a new bigger one, or copper plate its OD, or use Loctite.
Otherwise, could it be that as you say the engine has been rebuilt, the bevel gears are not perfectly adjusted? Any play there will make noise.
The good thing about Ducati single heads is that removing one to check the above is not a huge job.
The trickiest part is probably getting the bevel drive synchronised correctly on assembly.