One speaker hums and stops when touching back of speaker

I have a pair of Sdj-80x speakers connected to a djm-250mk2 and I'm having a problem that when I switch them on I get a humming sound from one of my speakers which stops when I touch the back (weird).  I've switched the cables around and the same speaker still has the problem.   I'm using XLR cables. 

Appreciate any help.