Mods; please move this to mechanics if it suits better but I thought I'd try the knowledgeable folk here first.

A bit long-winded I know so please bear with me.

The FD cable on one of my bikes snapped yesterday. It is internally routed down the down tube, over the bottom bracket and exits close to the FD mount. It snapped pretty close to where it would clear the end of the down tube so I got a bit suspicious about there being some kind of friction issue.

I fitted a new cable after a bit of fishing around. I took off the seat post to facilitate this and I think I can see how it works. Although I can't see it I believe there is some cable housing routed from near where the shifter is down to the end of the down tube. Above the bottom bracket there is no housing and you can see the bare cable when you look down the seat tube. The cable then turns upward and exits through a little guide near to the FD mount.

I fitted the cable and it works fine but here is the problem.

When I pull back on the shifter at a certain point the cable twangs just like a guitar string when plucked. It does it every time. I'm worried that the cable is snagging on something and then being released when the tension gets to a certain point. I think it won't be long before the new cable snaps and besides the noise is pretty unsettling when shifting.

Has anyone come across this before. I don't want to go through the hassle of refitting the cable again unless I have to.

I've attached a drawing to illustrate the routing at the bottom bracket.

Thanks for any help