A change in the air on high-speed rail?
Listen hard and you'll hear the screeching of wheels.
You can't do a U-turn on rails so, perhaps, it's the sound of a fast-moving train being slammed into reverse.
Labour insists that it has not abandoned its support for HS2 but it is now raising questions not just about the costs of building a new high speed rail line but the benefits too.
The official party line is that the party is just asking and being seen to ask the proper questions about a project whose budget has ballooned and the case for which has been re-written.
Supporters of HS2 predict that Ed Miliband will not abandon a scheme launched under the last Labour government.
However, it is clear that Ed Balls can sense the wind turning on this issue.
He is aware of growing public doubts. One of those close to him told me that HS2 is now raised spontaneously by focus groups as an example of money being wasted.
Most devastating of all, some even compare it with the Millennium Dome.
There are so many Tory MPs opposed to HS2 that the plans may need Labour's support. After today, that cannot be guaranteed.