A lot of good suggestions. But a tap that becomes a real knock is a concern. Most the common issues will not produce a grinding sound either.

Although I am a believer in cleaning out the fuel system from end to end before putting one of these in regular service. Shortly after acquisition.

I also I agree it is better not to start it. Yet it might become required to find the source of the noise. Your description for example could just be an alternator seizing up. Is the belt still on the air conditioning pump? On the 616 engines ninety percent of the time if there is an internal bearing issue it is the first rod bearing.

The more I thought about this the more I agree with the checking of the vacuum pump. When the engine stalled out on the next start something that was already getting bad just got really worse.