I have an NC1 and with the engine cold I have a repeating hiss noise from the top right (I think) part of the engine on tickover. Roughly 3 times a second.
Any ideas what it could be? Heater and AC both off.
Can’t really hear much difference in that sound clip.
So it only happens at tickover?
3 times a second is too slow for most things inside the engine as they are operating at roughly 6-13 times a second at 800rpm. Could be something on the fan belt, that would be in the right frequency for a full go around
As far as I know only tick over. Makes sense for it to be belt or something external related due to timings of the noise. Maybe a leaky vacuum pipe?
Sorry for the ridiculous questions btw. MX5 ownership is new to me. Ask me anything about an N52, M54 or S50 BMW engine and I can identify the slightest issue, but Mazda is still a dark art and I’m probably being overly cautious with my 109k mile NC1 
Does it stop when engine is hot?
I can’t make it out through that row, but if it’s rhythmic and increases with engine rpm then i’d start by looking at the drive belt and various pulleys etc.
Haven’t listened to it once the engine is up to temp, but will try that out.