Air handler ramping up and causing noise throughout supply ducts

Question

Have a new 2.5 ton heat pump (14 seer) and new air handler (Trane 2.5 ton tem6). All Electric. Our biggest issue is the noise coming out of all supply registers. This replaced a 3 ton TWE036 air handler and 10 seer outside unit. We have been told a lot of different things and it seems like where we live, no one does manual j or d type analysis. House is 2000 sq. ft, on a slab. Air handler in a laundry room with a 10 in base (downflow application) Supply ducts in the slab (10 runs of 6 inch corrugated black plactic duct), not a perimeter set up. Return in 4 bedrooms and one in main hallway (not noise in returns), told 2000 CFM can pass through the return system, which is basically a drop ceiling in shape of T. Where the return connects to the Air handler there are 3 openings feeding into the return plenum (6x18, 6x12, 6x10 approx.) Filter is 20x20x4 merv 8 above the coil Installing company says to create a divider within the base to minimize air turbulence or go with a TAM4 air handler possibly.
Another company said to add more return air, but did not specify how. Another company said it is a supply duct issue and to add a new supply run up through the attic to some of the colder/hotter rooms to alleviate pressure. Static pressure readings were done with two companies saying .25 and .18 of total ESP and the other company saying it was 1.13 (all readings based on before filter and done within base) Temp drop over the coil was 22 degrees This has confused companies and us as well and even one company thought it was the evaporator coil causing the restriction. I am not sure what is going on within this industry, but it is tiring on hearing how much experience everyone has and then they cannot figure anything out. Also, I wish contractors would not blame the ductwork. If the ductwork cannot be modified, then just propose a solution or the system to fit the ductwork. In addition, we had over the summer condensate drain backup twice as well water blow off into the blower compartment. We have tried all the dip switch settings, but nothing changes. Noise goes away when remove blow cover and just get air noise. Also, seems to be confusion on how much CFM can go through supply ducts as well as if corrugated duct is more or less noisy. Depends on contractor. Any direction would be appreciated.