Given that you've tried knockless injectors and it still knocks, it's more than likely a poorly seated crush washer on the delivery valve for that cylinder, or a worn out delivery valve that no longer seals properly on that cylinder. Air will be present in the return line until the car is driven long enough to burp it out. The return lines are "leak lines" and very little fuel flows through them, it takes a while to get the air out when you've had them off.

It's also worth noting that brand new or refurbished injectors can knock or nail for up to 1000 miles after installation as they break in.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]