Distorted / poor sound from single channel

Am running Rekordbox with 2 x cdj900 controllers and the one channel has developed v poor sound which which can only be described like a belt driven deck having a stretched belt (sounds like fluctuating speed).

Have developed the fault on channel 1 when using the controller to control Rekordbox but when I put a USB stick directly into the deck thus bypassing Recordbox it sounds fine!! further to this I decided to cross over the two channels within Rekordbox audio output settings so channel 1 became output 2 and vice versa..... this resulted in channel 1 now sounding fine with the fault moving over to channel 2.

Will be trying to re-install the software on the laptop to see if that resolves the issue but was wondering if there is anything in particular I should be looking at??

Thanks in advance.