Can I fix this?

I just inherited a G4 powerbook. It is an Aluminium with a 15" screen and 1GHz. As the owner several Mac desktop computers over the years I've found it pretty easy to use except the CD rom drive has a problem. When I put in a CD (i've tried music cd, DVD and software) it sounds like the drive is trying to read it but it hesitates and then ejects the CD. What is this likely to be? Could it be a setting that needs changing? Any suggestions at all? Would truly appreciate any advice. You can probably tell I'm a computer dilettante!!
By the way, there is a weird little button on the left side of the laptop which pops out if I press it. I can't figure out what it is - I didn't get a manual.
Thanks, Jude