Not all bends are created equal. A good bend has a gentle turn, and introduces no sharp edges to the airflow.
The first picture shows an elbow with a hard 90degree turn on the inside, indicated by the green arrow. This will cause turbulence, even at low speed.
At higher speeds you may even get whistling
This next picture shows another view of the sharp bend (green arrows). You can also see that the joiner doesn't provide a clean transition between the elbow and the pipe. Because there are only some small locating ridges (red arrows) rather than a continuous junction, the hard edge of the pipe (yellow arrow) is presented to the airflow
Much better. The inside bend (green arrow) is nice and gradual. This elbow also provides a full surface (yellow arrow) for the pipe to butt up to, making for a seamless transition
If you can't find a nice gradual bend, you can use several less severe bends, as shown in this picture, courtesy of Wolfgang Krieger from Munich
If you are stuck with one of the less than perfect bends, make sure you sand off any sharp edges
