For over a year my Felly has suffered from a slight but distinct resonance/vibration in the range 2,500 - 2,800 rpm (starting and ending exactly at these points and most noticable when decelerating). Gear (speed) doesn't matter, so driveshaft/wheel balance is ruled out. I've vaguely suspected loose air intake box or exhaust but haven't really bothered since it has stayed constant, not getting any worse.

Bur suddenly I noticed that the resonance has disappeared. Nice, but being the curious type I wonder why? A fortnight ago I had the cam belt and water pump replaced, so: could it have come from the pump?

Doesn't sound probable to me, surely the mass of the impeller is too small to create a resonance?

The garage also replaced a worn exhaust rubber mounting, maybe a more likely explanantion?

I'd say the latter - but then what do I know?

Prob was the water pump/cambelt if the car suddenly feels smoother.

At least yours still works.

Look for alternator bearing also alternator mountings.