I have a lock of the pictured design. The rubber rings at the ends of the U would be perfectly good for the job of pressing against the bar to keep it from rattling, except that they're not close enough to the bar to do that.

So I slid each ring out a bit and put on a small plastic cable tie in the space opened between the ring and the covering on the rest of the U, and that did the trick. The rings now firmly press against the bar when the lock is closed, and it doesn't rattle while mounted.

(Actually, it didn't work quite like that for the end with the lock mechanism, due to it and the rotating sleeve; instead I placed the cable tie outside of the rubber ring so that it meets the lock. I'd recommend not doing that and using something narrower to fill the space, if you can find such.)

The advantage of this approach over some of the other suggestions is that you are less likely to get stray material jammed in the interface between the U and the bar, because the supplied rubber rings are thick, close fitting, and otherwise suited for this job — just in need of a little spacer.

I have a lock of the pictured design. The rubber rings at the ends of the U would be perfectly good for the job of pressing against the bar to keep it from rattling, except that they're not close enough to the bar to do that.

I used rubber O-rings from the hardware store (found in faucet repair parts) as shims between the original thick rubber rings and the black sleeve over the main body of the U.

The advantage of this approach over some of the other suggestions is that you are less likely to get stray material jammed in the interface between the U and the bar, because the supplied rubber rings are thick, close fitting, and otherwise suited for this job — just in need of a little spacer.

(I originally used plastic cable ties, but those are too wide, have a lump at one point, and eventually failed under sunlight.)