Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
A key that is 'loud' (which is what I would call the problem you are asking about, in summary), can be fixed.
It's a matter either of the keyswitch mechanism, or something else local to the key itself (or that region of the piano's workings.)
The piano is out of warranty and thus warranty will not cover this problem, at 5 years of age.
However, there is a variation between any cost estimate, for one mode of getting it fixed, and another mode.
- your choice is either to try to fix it yourself (cheaper, but much harder to do), or to have it serviced by a local, quite competent professional at Kawai piano repairs of this sort. (And, this is a very easy fix, and routine, compared to what piano restoration of an antique would involve.)
To fix it yourself might cost something on the order of (aside from the time spent on it, which I cannot estimate) maybe $35 to $75 at most.
To have it serviced by e.g. a 'housecall' (or two perhaps, if the problem is not described to the piano tech in advance in some detail... so go into detail if you are asking someone for servicing, as you have gone into detail with me), would really be up to the servicing professional's charge rates... but I would say something between $60 and $250 would be reasonable to expect.
The key thing is, 1. to describe the problem to the piano tech or techs who are competent for this job, in your region, and 2. to then ask them, before engaging their services, what their estimated charge to fix the problem would be. That's the surest way to know what it will cost, that way.
So, please tell me in Reply, what region of the world or this nation, you are located in, and I will give you a list of Kawai piano repair pros to try contacting (or, for sparsely populated regions, those who might be Kawai paino repair pros). Just let me know your town/city and state, or your zip code, and I will give you those listings to check on and contact. Thanks.