So, I have been browsing this forum for years as part of my research on geothermal. My old furnace but the dust so I finally took the geo plunge. A little bit about me, I'm a mechanical engineer in the thermal fluids area, I understand hvac, airflow and all the related processes, but don't mistake that for me saying I'm an hvac expert. My system is a climatemaster tranquility 30 four ton unit. I drilled for my loops, 200 foot each. Variable speed flow center controlling the loop from temperature delta. Functionally everything is working great, AC heat rejection to the loop is right where I would expect it to be. But, my real problem is crazy noise coming from the supply plenum. I had completely new ducting put in my entire house. The contractor who did this work did what I would call a "splat" duct where the 18" x 18" plenum hits directly into the 8" x 32" trunk line. He did come back and put vanes in the top, but I don't think it has helped that much. I do have the service tool and it is indeed set for 1400 cfm on stage 2 cool. The pressure delta from the return plenum to the supply plenum is between 0.07 and 0.1 inches of water. I have a manometer on that all the time. If I open the cover to the fan, the sound goes away. If I take the return off it is still there. That is telling me turbulence. I'm asking opinions to see if experts here agree with me. The original contractor is out of ideas and another contractor I received a second opinion from didn't have any ideas either. Thanks for your time. Eric.