what is this sound

I've heard that before on occasion. I don't think it's a problem. Maybe it's the DEF system pulsing fluid?

 

This must be a detroit. I had a freightliner with detroit that did this non stop..

 

O.K. Now we are getting somewhere! What you have is an injector(s) that have a cracked seat face. Where the injector meets the injector cup face there is a copper washer. Under the washer the injector nozzle seat has a crack. I have dealt with this "chirping" many times. Detroit solution is to replace all 6 injectors since it will probably eventually happen to all of them anyway. Mucho $$, so you can pull and inspect, sometimes magnifying class is needed, and replace only defective ones. Good news is Bosch identified problem a while ago and new injectors don't have this problem.

 

My 2016 and 2018 have both made this sound since new.

 

This is the sound I think you are referring to, and would make since. A cracked face on an injector will put air back into the fuel system. The sound I hear in the op’s post is the fuel quantity control valve doing its thing.