Crackling sound after an hour on Traktor S4

So I have done three DJ sets in the past few days, each time using my MacBook with Traktor S4 software and the Traktor S4 controller, each time in different locations with different set up. All works fine for the first hour and then gradually a crackle starts to appear, building, getting worse. The only solution is to switch everything off after which it is fine ... for 45-60 mins and then it gradually comes back. Latency was set at 576. I increased it to 1216 but the crackle still returned. Otherwise everything is fine! Any thoughts?