Cog spacing problem on Ultegra HG800 cassette

I recently fitted an Ultegra HG800 11-34 cassette and new 11-speed chain to a bike. When the chain is on the third-largest cog, in either chainring, there is a slight ticking noise every second revolution of the cassette. All other gears are completely silent, or as silent as a chain drive can be, and the derailleur is correctly aligned under the cassette cogs.

With the bike on the workstand and the chain on the offending cog, I can see that as the cassette rotates, there is one of the teeth on the next smallest cog (i.e. the fourth largest) that just catches the outer plate of the chain as it comes around, and the ticking noise is the chain hesitating and then snapping down past this tooth and onto the cog. It only happens every second rotation of the cassette because the third largest cog has an odd number of teeth (27), and it's therefore only every second time around that this problem tooth meets the wider outer plate of the chain rather than the narrower inner plate.

I can't see any runout at all in either of the cogs when rotating the cassette with the chain elsewhere, nor does the tooth that catches look as if it is bent inwards at all. I'm wondering if it's of any significance that these two cogs are on different spiders - the three largest cogs are on one spider and the next three on another, so the spacing between the two cogs is dependent on the spacers built into the spiders being the correct size to give proper cog spacing. At this point I'm wondering if I should try to cut a very thin shim out of something like an aluminium drink can, which are around 0.1mm thick, and fit that between the two spiders.