If you read the information at Why do I hear pops and clicks in my audio tracks?
, you will find instructions to confirm that DAE (digital audio extraction) is being done cleanly without errors.
This doesn't sound like a DAE problem, because you are having the crackling sound only in the last tracks. This would seem to indicate a problem in the burning process. Since you can burn only at 4x max, burning at 2x may be worth a try. Be forewarned though, that the latest high speed rated media may perform poorly at low burn speeds.
What media and speed rating are you using? I would try a different media brand, preferably one that is not rated for the highest speeds. It may be hard to find media like the older 1x-12x rated types, but this may be a better match for your burn speed.