Intermintant Starting

Hi, My dads 99' or 00' grand prix Gtp suppercharged somtimes starts fine. But somtimes when we turning the key to start it nothing happens. You can hear a click and a buzzing sound when the key is turned to start the car. All lights come on also. This happens sometimes. We had this problems 7 months ago so we replaced the starter. But I guess that wasnt it. Can anybodyy give me any advice on what to do or look for? THanks.

Welcome to AF.

How old is the battery? Get the charging system (alternator) and battery tested. Also, check the condition of the battery positive and negative terminals and cables. And to the underhood fuse and electrical box. With car off and at idle check the 12 volt output.