@37K miles my 2012 Fit Sport AM has developed what I think is additional valve noise. It is unnoticeable at idle and at higher speeds, but when coasting at very-low-but-not-zero throttle, or accelerating very slowly, there is a multitude of rattles from ahead of me. Sounds like tiny marbles on a gently shaking steel pan.

It's still very quiet through. For example, I'm sure darling Wife has not noticed a thing yet. So my question is: When should I be concerned? I'm not afraid of pulling the top of the engine off and doing the valve adjustment (and presumably spark plugs if it requires taking the cowl off). But I AM afraid of going to all that work and finding out everything is still in fine condition. Should I wait for this to get worse? Will it get worse, or will something break suddenly?

This is pretty low mileage to have trouble with valves ... We do not put a ton of miles on the thing.

 

I'm betting it's a rattling exhaust heat shield or something else similar.

Valve adjustments are definitely a major important part in a good running Fit, but you're just not anywhere near the miles that would require one yet.