I have a double, undermount stainless steel sink attached to a granite countertop. The smaller side of the sink has a garbage disposal attached to it. It was installed fifteen years ago. About two years ago I noticed a creaking noise when walking or shifting your weight when in front of the sink. It is hard to tell if the noise is actually coming from the sink or the connection to the garbage disposal. If I press down hard on the sink it stops the noise. Could it be the sealant between the sink and counter has broken down?

It could be a lot of things from the things you mentioned to anything on that area being loose and causing two surfaces or objects to rub and squeak. You'll have to try to figure out what's loose by trial and error.

The root cause is the floor. Look under the sink, we can't, and try to find the noise while someone is shifting their weight. Could be metal or plastic coupling.