High pitched hum and whine while extruding  

High pitched hum and whine while extruding

It seems this MK2.5S was inspired by that 1988 U2 album. The 'Rattle' comes from the part fan as described here:

https://github.com/prusa3d/Original-Prusa-i3/issues/91

But that's a different issue. At issue here is the 'Hum 'which seems to appear only when extruding, and when doing short infill segments it sounds almost like spitting:

Slic3r is using the default MK2.5s configuration. I tried adjusting the idler screw, no appreciable difference. Extrusion appears regular on the first couple layers but I have lost far too many prints due to stringing, globbing and poor layer adhesion than I ever did with my original MK2S, which was a problem-free workhorse for over a year. I'm almost ready to scrap this extruder and go back to the original MK2S one. I haven't been able to find any loose screws or other problems with the assembly.

Can anyone help me identify this awful noise and suggest a fix?

Thanks for any help...

Van