Early Spectrum C40-8

[rockymidlandrr]

I have happened to acquire a few of these here latelyand the last one is in pieces and getting a decoder installed and the other 2 have gone through this process.  In hind sight and before I wrap this project up, what can be done to make these beasts run quieter?  They seem to have a growl to them and it's very noticeable at high speed. 

Just looking for some tips and pondering while I can't work on them...