If it's the model I was able to blow up a picture of, it appears to have a custom grommet and not just a simple rubber doughnut. Your best bet is to do it right and get the appropriate part from a dealer, disconnect the cable housing from the lever after removing the cable and slide the grommet down into the hole in the downtube. Pain in the butt, but it will probably fix the problem and look a lot better than a cobbled job.

I don't know the specifics of your bike, but for typical cable downtube rattles, you can wrap the hose in some foam tubing (pipe insulation). If you have an access hole in the BB area, you can put 2-3" around the hose, tape the slit closed at the top, and slide it up the down tube. You should also tape the bottom closed.

Did exactly this on my Giant Propel. Worked a treat.

Hey trimule, I was afraid of that. I think you're right. Gonna contact Ridley and see if they can send the part.

Not sure there's enough room to get insulation around the cable and then slide it into the opening on the tube. I saw somewhere else that some people glue them in. Was thinking that some black electrical tape wrapped around the cable may tighten the cable in the opening?

If you don’t have room to get an insulating wrap through the frame holes, you can try cable ties. Zip them tight around the cable but don’t trim them. The excess tie end will then curl itself to the inner profile of the downtube & hold the cable still while riding.

I guess the solution depends on if it's rattling around at the opening of the frame or at some point inside the frame. So my first step would be to find that out. I'd try something quick and temporary like electrical taping it a bunch at the opening, not trying to make it neat at all. If it's gone after that then you know it's worth thinking about a good long term solution at the opening. If it's still rattling then you are looking for something internal like the zip tie idea.

You might even figure out which one it is by shaking and tapping the frame a bit.

Thanks folks! I think it's rattling from the cable rubbing up against the opening, but I'll confirm with the tape idea. Have a good one....

So... I wrapped electrical tape on the cable housing at the entry and exit holes on the down tube. The cable still rattled inside the tube. I played around with how much cable was in the down tube. I was able to get rid of the rattling once the housing remained in constant contact with the inside of the down tube. The electrical tape held the cable housing there. Hopefully it will stay like that. Moral of the story: I had noice from both the hole where a grommet should be as well as the rubbing inside...

If you're still getting downtube rattles from the housing and can't fit foam insulation in through the bottom bracket access areas, you can get polyester pillow batting/filling and use a chopstick or long dowel to pack it up into the tube to surround and dampen the cable movement.

I finally went for a ride. The electrical tape solution worked (so far). Actually doesn't look bad either....