Home button isn't clicking, but is still slightly responsive.

I just bought a used iPhone 4, and it is not under warranty any more, or free repairs are out of the question.

The home button works, but not not all the time.

Sometimes when I double click, it only registers as a single click. Sometimes I have to press harder to get a double click.

I compared it to my buddies iPhone 4 and his home button has a definite "click" or "pop" feel when his button is pressed. My phone definitely doesn't not have this same feel.

Here's my question. I'm looking into replacing the home button myself, but I'm not sure if the replacement button is responsible for the "click" or "pop" feel, or if it's the dock connector, or another part.

thanks for any insight.