It does sound abnormal to me, like the endmill is bouncing on the material while cutting. Is this a fresh cutter?
note: to be honest I did very few cuts in aluminium with the stock Z axis before I upgraded to the HDZ though.
To confirm whether there is a mechanical issue I would maybe run a test cut in the exact same conditions as what Winston advocated in the aluminium series of videos (here)?
EDIT: I’m sure your wheels and belts are fine, I was thinking of that kind of wobble (just more subtle):
77861282_1535429133262942_8184613103732785152_n.zip (854.7 KB)
(it’s an example I downloaded from the Shapeoko’s FB group, rename file to “mp4” to view)
EDIT: to get a feel for what “normal” sounds like, check out the many videos in @Vince.Fab threads e.g. that one
videoplayback.zip (835.4 KB)
(again rename to mp4)
or that old video of mine I grabbed while running an adaptive clearing toolpath. All of these are probably done with an HDZ though, but you get the idea.