Just took a wander down to the bike locker - the valve stems don't rattle if the wheel is shaken or dropped or anything however, if you grab the stem with your fingers, you can move it around. I suspect though, that the tension in the rubber tube would prevent any real tapping, particularly of the frequency I'm getting. The rims are Velocity Deep V so there's a fair gap between where the tube actually sits and the hole in the outer rim and this might affect things too.

I had hoped to check the cassette for obvious looseness but that wasn't going to happen without removing the wheel and seeing I didn't have the right tool to tighten the thing with me, I didn't bother - on the list for next effort at home though, regardless of whether other attempts fail.

Richard