I have a ~15 year old, 5 ton Carrier A/C unit. Recently the circuit breaker tripped when I tried to turn on the A/C. After that there was no power to the unit even after I reset the breaker. A technician checked it out and said that the circuit breaker was bad and it needed to be replaced before he could diagnose the A/C unit. After the circuit breaker was replaced with a new one (same specs as the old one), the compressor now turns on but the fan does not. However, the compressor AND the circuit breaker now make a weird noise. It sounds like a cranking, oscillating noise. I have to turn off the circuit breaker manually. The technician thinks that the compressor is bad and needs to be replaced. What concerns me is the noise that comes from the new circuit breaker when the compressor is on. Is it a problem with the new circuit breaker or the bad compressor could possibly cause this noise? Thank you for your advice.

It could be
A high current draw on the A/C circuit that the breaker was replaced on.
It could be loose wiring connections in the main electric panel or at the disconnect/unit outside.
The HVAC person should have taken a amp reading on the circuit to see if it is being maxed out by the A/C unit.
No technician should THINK the compressor is going bad. They are supposed to have the proper tools and know how to use them to bring the problem you are having to a conclusion by a statement that the compressor or other item in the electrical system is causing the problem. It could be the run capacitor for the compressor has lost some of its value and that is causing the compressor to labor more when it is trying to do its job. This is something a true professional would also test.

You say the fan does not turn so it could also be a outside condenser fan problem OR it could be the capacitor for the fan motor that is defective.

Depending upon the manufacturer of your electrical panel you could have some stressed lugs or circuit breaker distribution bars. This will also cause a high current draw on the circuit. Loose connections are a killer for A/C circuits.

As you can see there are quite a few areas that can cause problems. I suggest you contact a real service person who knows there stuff and does not guess. This way you will know the truth about your problem and can choose your option to repair or replace.

Thank you hvac1000 for the quick response. I couldn't agree with you more. I've scheduled another technician to come out and look at the system again. Thanks to you, I know what to look for now.