I have a mixte I purchased used. It is a single speed. I've put a new saddle, handlebars and headset on it. Sometimes when I'm riding it starts creaking. It doesn't matter if I'm coasting or pedaling and it doesn't happen all of the time, but once it starts it seems to go about once per wheel turn. I can feel it in my seat, and it seems to becoming either from under me or behind me, but it's steel, so I can't pinpoint the noise because it travels so well through steel. I've watched the wheel and there isn't anything moving when it creaks so it must be internal somewhere. I'm thinking it's the hub or the bottom bracket. Is this possible?