Nord Lead Rack 2 Master Level buzzing (??) and other problem

[jjjjhhh]

Hello Nord Forum! i bought Second hand Nord Rack 2 in 2019 October, and suddenly when i tweak the master volume to right, sound clips (or buzzing, i don't know how to explain this
) and sound doesn't get louder, and it goes quiet. i would like to request a fix, but i am afraid of high-cost fix.. so i am asking this forum, if this kind of thing happened to you before,
and how much would the fix cost, in south korea..

and 'other problem' is when i turn on my Nord Rack, i can hear some buzzing noises in back panel. can you help me about this?
thanks,
j