(So I just got an almost mint skoda felicia with 46k miles, very minimal surface rust on wheel arches and a rust hole on the left front wing as it was bumped into by the previous owner [will get sorted soon hopefully], apart from that its amazingly clean.)

So when the car idles or is at an rpm where the engine vibrates more than usual, there is a knocking noise coming from behind the engine. It isnt coming from the engine, it sounds like a mount because its a bit muffled, and doesnt sound like metal on metal. Does anyone have a guess of what it might be? And if its engine/gearbox mount, can it be done with just a jack or is it a ramp job?

Thanks in advance! 

I suggest raising the front of the car on two jack stands to locate precisely the source of the noise. You have full access to the car while we count only on a vague description.

Anyhow, given the low mileage, I doubt there is something wrong with the middle engine mount. Original parts last very long on normal driving and maintenance. I am more inclined to believe the noise has to do with the exhaust system.

I don't recommend working under the car using only the jack.

@WaifuRacer - I agree with @RicardoM  that the noise is quite likely to be exhaust related. If there's a decently run local garage or fast fit place near you, they'd help with inspecting the hang of the system, and for broken hangers.