Joint cracking

Hello,

I have a 4 year old shih tzu / cavalier king charles spaniel cross named Leon. Lately I have been hearing one or both of his knees, front legs, cracking.
I took him to the vet and got him checked over - they got him to walk (but looking him from a distance to see if he was leaning) and felt his joints. They did hear a crack when the vet lifted his front paw, but was unsure if it was Leon or the vet. Although his paw has cracked when I lifted it before. They didn't think it was a joint problem and believed it could be a nail clicking, but I also hear the clicking on the carpet. So you shouldn't hear a nail click on carpet, right?
Leon walks fine and runs fine, the odd time he lifts his paw. But I am not sure if it is due to joint pain or that he just decided to lift it for no reason.

Not sure if I should go to different vet for a second opinion, is it OK to do that? I am just a little concerned for him that it could lead to Arthritis later on

Thank you

It is definitely okay to go to another vet for a second opinion if you aren't sure, or aren't satisfied with the answer you get from your regular vet.
And you're right, you won't hear a nail clicking on carpet, since it is soft there isn't anything for it to click against.
If he is lifting the same paw, and that is a new behaviour, I would be concerned. It doesn't hurt to look further into it!
My older dog's bones crack sometimes when he gets up, but he is 9yrs old and almost 95lbs (not fat, just a big dog) so I think that is more normal, as he does have some arthritis. I'd be a bit concerned since yours is young.
Hope Leon is okay!