Honda has issued a Service Bulletin on the 1994 VF750C Magna regarding a knocking noise from the cylinder head areas. The noise varies as you tip the bike from side to side. It is likely caused by sideways thrust movement of the camshafts which then knock against their holders. The solution is to drill a small hole of precise diameter, length, and location, in each of the cam holders. This allows oil to seep into the space between the end of the camshaft and its holder to cushion the sideways thrust motion.
In the USA, this service bulletin is "VF750C #1". In Canada, this bulletin is "II-1-94", dated September 21, 1994. Your Honda dealer should have a copy of this bulletin.
You may view an image (60 KB jpg) of this bulletin. For a bit more detail, here is an enlargement of the diagram from above bulletin (75 KB jpg). Finally, someone else on the web has provided yet another diagram for the cam knock, this time on Honda letterhead.
