despite the snark, junque has good proceedural advice, if you have an injector that is out of spec and 'nailing', with the car running, cracking the line @ the injector will disable it -smooth out the sound-until you tighten up again.

The car may idle rougher for it, but you are listening for change/reduction in the clatter. Do each one in turn to isolate a bad injector(s). If the relative change is the same across all 6, then that is 'normal' or all are out of spec. Testing the pop pressure is the only real way to know for sure.
It could be that what you are hearing is normal and the key is to note any change over time.

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1991 300D