Hi ubs28,
Most folks who own the Shure 846 and the Mojo combo are on both sides of the fence. One side claims a slight audible hiss that isn't detrimental to music enjoyment, while the other camp claims no hiss at all.
YMMV but it also depends on how sensitive to hiss are you? It is unfortunate that you hear hiss whilst under the impression of other owners that the Mojo doesn't hiss. This issue of hiss has been debated multiple times already and at the end of the day, it comes down to 2 things:
1) Is the hiss bothering you or is it a bother when you're enjoying the music. Does it get in the way of your enjoyment?
2) Is the hiss only with your SE846? If it isn't then it could be a faulty unit. If it is, then read point 1)
Cheers. Hopefully you can sort it out and enjoy what most of the users here are experiencing.