Yes, sounds like the HDD to me. I replaced the HDD in my old G3-500 iBook (original SnowBook) and yes, it was "challenging". You should still consider it though, but be prepared to almost completely strip down your iBook.
Not knowing which model you've got, have a look at this link at iFixit
If you do embark on this exercise, you will need an "older" style ATA HDD, which new, are relatively more expensive than today's SATA drives.