You need to either tighten the p/s belt or replace it. This would have happened whether you changed the alternator or not. The belt is simply worn and needs tightened up or replaced.
Your car is soon to be nine years old. It is time to replace the belts and hoses if it has not been done...
I got a similar problem with my pendant lights, Its likely the 2 reds are live, the 2 blacks are neutral, and the single black is a switched live
So, for a new light you are likely you have a brown and blue wire.
The two reds stay together
The single black goes to the brown in your new...