After checking the service manual I found out that my previous post is not correct.
The headphones and speakers have separate amplifiers. Also woofer and both L+R speakers (midrange and tweeter) have separate power amplifiers. You say that the crackle is only in the left speaker (mid range and tweeter). If it is not in the woofer then you will probably not hear it In the headphone either, also not in the line output. Woofer and other speakers partly use same preamplifier (equalizer). This means that the source of he crackle probably will be in the power amplifier of the left speakers or in the speakers itself. You should check the presence of the crackle in all available outputs (sub-out, line out, headphone, woofer, midrange sp and tweeter) to find the source of the crackle. Good luck!
Bert

Please notice that my previous posts are related to the KN7. I do not have the service manual of KN5 but I assume that the basics are the same.

Hi, your problem is with a KN5000, the service manual in on this web site go to Nigel's archive and then KN5000 the manual is there but if you have been using the KN7000 manual the KN7 is very different in all ways as the amp system is all digital right up to the last stage of amplification as the keyboard is able to transmit and receive via USB on the audio side.for your keyboard use the correct manual as all KN series keyboards have major differences between each other.
Best regards John.