Loud noise like a rod or a piston when driving interstate speeds.

SO When you mention the transmission is new and its still making the noise, Im guessing you replaced the transmission thinking you had a trans problem? Who is the guy that just swapped it out to hope the noise was in the trans without checking? so anyhow, new trans and same noise, Varies with speed as you say, Rod knock sound, Check your carrier bearing and pinions and angles. Start there

 

What engine? Because that’s pretty much exactly what an isx Cummins is supposed to sound like imo

 

Don379 is on the right path. Check the rest of the driveline. A simple thing like a u joint can cause that kind of issue.