I get a very low level hum that seems to be related to the mouse.  It is just a usb mouse.  any insights on this?

I would suspect this and your other problem may be related.

Check for a ground loop.
Check all USB cables.
 
In the past, I've seen a poor quality USB cable cause noise when connecting a Motif ES7.

fireberd
I would suspect this and your other problem may be related.

Hmmm.  maybe.  I don't have any other sound drivers enabled.  I thought it perhaps was the hdmi sound.  I disabled that.  

Jim Roseberry
Check for a ground loop.
Check all USB cables.
 
In the past, I've seen a poor quality USB cable cause noise when connecting a Motif ES7.

I suspected a cable shielding issue.  the mouse is a fairly cheap usb one.  the usb cable that connects the motu 1248 is decent 

Oh. and just to clarify.  I don't have the mackie onyx hooked up on this machine
 

Buy a cat!!!
 
Naughty mouse or a ground loop
Try another mouse would be my first port of call and then ground loop next.
 
 

igiwigi
Buy a cat!!!
 
Naughty mouse or a ground loop
Try another mouse would be my first port of call and then ground loop next.
 
 

hmm how does one find a ground loop?

You can go thru your entire setup cable-by-cable (with headphones and the headphone-amp opened up a bit)... to find any culprit.  If you do this, be careful with your hearing (no surprise bursts).
 
The easy way to avoid ground-loops is to power your studio gear from a single outlet.
Powering from multiple outlets is virtually guaranteed to cause ground related noise.
Balanced cables can help combat this... and may make the ground-loop inaudible... but the ground issue is still there.
 
If the sound is gone with the mouse unplugged, I'd get a different mouse.
Poorly shielded USB cable can cause audible "digital hash" (grainy sounding noise).