I just had a Bryant condenser installed and it makes a constant very loud buzzing sound when it is on. I called the service man and he said it was normal to be loud since I didn't get a new inside unit as well. Sounds fishy...

YEP it sounds fishy to me.

It was also stupid to not replace the inside coil because you are not running a efficient unit and it might use a bit more energy than the old unit depending upon the SEER rate and metering device.

This does not mean that the unit should be buzzing unless the refrigerant charge is off and then it could be a possibility.