The 300 series of mixers use grease in the trans case. I have never seen this type of noise come from the gears.

I suspect the noise is either from the motor or planetary. The first thing I recommend is to use a long screwdriver or socket extension -- put one end on the unit, and the other against your ear. Test different spots on the mixer and narrow down the location of the noise. Like I said, I expect the noise to be the motor. In particular the front motor bearing.

I can help you futher if you narrow down the location of the noise. Here is the parts catalog.

http://www.hobartlink.com/extranet/extranetmanualsandcatalogs.nsf/41B8C834DE2B5C6B852568E1006A9A01/$File/F19154%20Rev%20C_lk.pdf