Just another thought - have you told the landlord about the noise from the neighbours? Depending on who they are, they may be able to do something about the noise (i.e. if they own the neighbours property too, he could ask them to shut up or evict them at the next renewal). If they know about it and couldn't care less then definitely tell the prospective tenants.
I would keep out of it as they could mention it to the LL and then as stated by another poster, there could be some comeback on you. Who knows they might have some bad habits themselves!
What kind of noise do they make? Is it dogs, music, TV, heavy feet or shouting? I would definitely have to tell them as I would want someone to warn me if I was thinking of moving in somewhere. It's up to them if they think that particular noise would be bearable. It can seriously ruin your quality of life if you live next to unbearable neighbours.
Why do you care? Your apparently moving out. If you ruin too many possible tenants, the LL may find a way to hold you financially responsible for lost income.
DEFINITELY! its good-neighbourly. i would definitely want to know, and would certainly do the same for others - seeing as i am currently in that scenario!
I would definitely drop subtle hints to the interested tenants. I hate noisy neighbours (having lived next to a dog barking all bleeding day in the uk), I am sure they'd appreciate it.