Fast idle in neutral

I would suspect either a bad idle air control valve or a vacuum leak (specifically the intake manifold gasket) if there is no service engine light. If there is a light, get it scanned and let us know what the code(s) it has. You can get that done at the local auto parts shop.

Guide on how to check for vac leaks: https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Guide on idle air control (IAC) valves: https://www.2carpros.com/articles/how-a-idle-air-control-valve-works

How to replace an IAC valve: https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

And then I found this. It could be caused by a bad connection at the throttle position sensor. Here is the service bulletin released by Mazda.