The backpressure and turbulance of a right-angle bend is huge in comparison to a radius bend (dont' quote me, but I think it is something like 5x). Part way in between is to put an angled deflector vane in. Another thing often called for but often omitted, is to use a decoupling band to separate the ductwork from the vibrations of the air handler/furnace from the ducts. It is often just a 2" or so piece of vinal coated canvas connecting the two halves of rigid metal ductwork. This will go a long ways to isolate noise propagation into the ducts.