New pair of DJ50X soeakers - one of them has a buzzing noise

Hello

I recently purchased a pair of DJ 50X speakers. I have them connected via PCA to an m-audio delta audiophile sound card.

One of them emits a low erratic buzzing noise whenever switched on. There is no pattern - it sounds a bit like morse code but with a buzzing tone. I initially thought it was the cable, but even if I switched to new cables, or just swapped the cable with the other speaker I get the same noise on that speaker. 

Would appreciate any ideas or help!