Rekordbox Performance with CDJ 2000NXS/DJM900NXS and Mac

I have recently set up my CDJ 2000NXS/DJM 900NXS with Rekordbox Performance on my Macbook pro.

I have some sound issues as in at time it is crackling which doesn't sound good!

I don't have anything else running at the same time. I have and the highest headroom level set, but on the levels on the internal mixer look high (but not adjustable)?

Any advice welcome.

Thanks,

Marc