My 2.8 Month Old Grandson Complains Of Certain Bones Cracking.

My 2.8 month old grandson complains of certain bones cracking. Last night he said his feet were "cracking" so I rubbed them for about 10 seconds then he said it's better now grandma. He woke up in the middle of night and was crying about it but said it did not hurt. This afternoon he said his hand was cracking so I rubbed it bout 5 seconds then he said it's ok now. My son (the father) says he (the toddler) has been complaing of different parts of his body cracking for the past several weeks. I've been searching the Internet for info on this and really haven't found anything about toddlers complaining of their bones cracking but found nothing but a good explanation of growing pains. What would a cracking explaination be from a 2 yr old?