you can get a chain form any (japanese) bike shop. i believe that chain is a 525, but the parts guy at the shop should be able to look that up and make 100% sure. basically the pitch of the chain doesn't match the pitch of the sprocket anymore due to wear on the chain and sprockets, hence the "pop" you're hearing. i never noticed it when i rode it because the chain was loose. i should have tightened it before you picked it up, but like i said, i got caught up in customer jobs. "technically" it's best to replace the chain/sprockets as a set, but there was hardly any wear on the sprockets on the bike, so if you're looking to save a bit of money, just replacing the chain will do the trick. that's up to you though. either way, let me know, and i'll take care of it for you.

i just got your voicemail too (wtf is up with my phone??).