Hi All
Looking for advice about a noise under load in 2nd some times 3rd feels like its not in gear for few seconds with a lot of grinding noise, then its fine and will not do it again for days 
Thinking could it be loose comp nut or should I look in the gearbox ?
Trying to get direction before I pull it apart. Bike is 99 fatboy evo 40,000km
Thanks in advance
Cheers

I would be leaning towards the clutch.. 

box may need adjusting in side at the selector paws/or worn or clutch adjust if it only every now and then may be change to redline heavy oil have seen that fix a few shift change problems in car boxes never used it myself in a bike but some guys around here swear buy it

plus what stinky said if you know what pinging is  maybe your changing at too low of rpm   rev it dont lug it