I hate a rattling bike. My carbon synapse has been making a lot of noise on anything but smooth roads and those are few and far between around here. The bike has an internal cable for the rear brake and I thought it might be that. I've tried to apply brake to tighten the cable and see if that solves it but no change. I'm wondering what else it might be. One thought is the cables that run along the head tube and could be slapping there. I have checked bottle cages and it's not that.

Anyway have a similar issue and how did you solve it (if you did)?

I would load the bike the way you ride it with saddlebag, bottles, other attachments, lift it 6" off the ground and drop it, listen and try an figure out where the noise is coming from. Possible places I can think of are headset, cables close to each other in the handlebar area, cables entering/exiting frame, cables inside the frame, plastic cable guide under the bb, saddle rails to saddle interface, loose spoke, inner tube valve loose around the rim.

What rishardh said. It's hard to find the source of a noise when you're on the bike, but you should be able to isolate it that way. Also, be aware that noises can "telegraph" so they sound like they're coming from other places. I rebuilt a bottom bracket one time for a creak that was coming from a loose saddle clamp.

Well, based on my best guess the noise is from cables. Either slapping against the head tube or where it exits the top tube. I wrapped tape around the cable on the top tube to see what effect it would have. Seemed to cut about 70% of the noise. I'm thinking the rest is still the cables as they pass by the head tube before heading down the down tube.

Problem is, I don't know how to fix. Tape around the top tube is lame and pretty ugly. Can't tape the cables to the head tube as that would impact steering. Anyone know a reasonable solution? Thanks.

Maybe you got too much cable slack... you only need to be able to turn your handlebar 45 degrees either way. If this is the case you will have to shorten your cable housing. If you are doing this pay attention to how the cables are routed and arranged when it exits the handlebar and enters the top tube. Try and keep the cables away from each other and the frame.

Did you eliminate a loose headset as the cause? Sometimes it's not obvious. You will have to hold the front brake and rock it back and forth to see if there is any play.

Not sure if this is it but I just read a thread about the synapse having a bb30 and those have creaking issues. Usually you can tell if its that if it happens somewhat in sync with your pedalling. If so I would pull it, clean it, and relube it. Good luck.

Not sure how to determine the right amount of cable slack but it's the amount the bike came with. Less, I suspect, would make it tighter so maybe less to slap around.

Pretty sure it's not the headset but I can give it another quarter turn and see what happens.