I have been extruding for a while with 3devo Next v1.0, and whistle noises came from the screw system, even when it is filled with material. Does not matter the material used (PLA, HDPE, LDPE, ABS, etc.) The same whistle noises can be heard (link: https://files.ugal.ro/s/5KiZkB4hO8s2qVx ). It might be an axial off-center on the screw or a gripped bearing. The question is how to detect & solve the issue and if is existing any maintenance manual to disassemble/repair the main screw.
This sounds very familiair to me, since I have been working with these machines for quite a long time. First of all, this is nothing to worry about and it won't damage the machine or affect its performances. But, it can be quite an annoying sound.
A potential fix here would be to loosen (or tighten) the 2 bolts closest to the hopper opening. This slightly lifts the hopper + extrusion system.
Another option would be to remove the panels from the machine (by removing the bolts), so you can have easy access to all internal parts. Then you will see 4 bolts that connect the extruder barrel with the chassis of the machine. Loosening (or tightening) these might fix the issue as well.
Please let me know if this works!