350 Mack single axle dump....when u get it under a load it sounds like a horn coming from turbo...any thoughts on why?
What year and model? If it has a boost gauge how much boost is it getting?
Very good question! I'm just going to make a wild guess that there is a bad boost leak in the pressure side of the turbo. This is one time having the truck on a chassis dyno would sure be handy!. For starters I know I'd be looking over hoses, piping and if there is one the intercooler very close, basically anything that would be on the boost side of the turbo up to and including the intake manifold and it's gaskets.
Hole in the hose to the intercooler is most likely
2003 rd688
I always preferred the old tip turbine air to air intercoolers, large and cumbersome, in the way but pretty much bullet proof.
Check the hose clamps on the intercooler rubber hoses.
You can take it to a truck repair shop and have them test the charge air tubing for leaks. They just plug both ends at the first and last boot and pressurize it.
I just stick the appropriate size PVC slip plug in one end, and a slip bushing with a little fitting in the other, then use my radiator tester. (Or you could use a bicycle pump, or regulated air turned down to about 5-10 psi)