The last two years my AC has needed more coolant needed. I was told it is a slow leak that can't be found. AC works fine except for very loud bang at the condenser when it goes on. It is a Lennox unit installed in 2000. I am a widow on a fixed income, and will be selling this house next summer. I don't know if I should replace it or if it can be fixed. One service told me they don't know what is making the noise. Should I get a second opinion. I'm afraid a home inspection next year may catch this. Thanks in advance for any help.