Had a 317 for years, and never did something like that, so can't really help you. But you could try to separate the pre and power amp, at least you'll know in which amplifier stage the fault is. Maybe someone else can take it from there?
NAD 317 trouble HELP
I have been having a problem with my NAD 317 amp. Seems that the left side of the power amp has an issue, where out of the clear blue sky, it makes a nasty static noise, and will eventually trip the protect circuit. The condition is independent of any setting, be it volume, input, etc. The real kicker is that I have had it in the shop (Saturday Audio) twice now, and the thing won't act up for them! Anyone else have a similar problem? What was the fix? Any hints would be GREAT! THANKS!