After I've been driving my Elky for a while, it makes a strange noise, kind of a loud pop, though it sounds at times like a hiss. It does it every 5-10 seconds when the car is warm and when I stop at a street light. It seems to be coming right from the other side of the firewall and I haven't been able to reproduce it with the car in park (Maybe it only does it in drive? Maybe it only builds up pressure when I'm moving?)
I recently disconnected most of my AC stuff. It seems like it started shortly after that.
Maybe something with the power brakes (everything there is less than 6 mos old and less than 1000 miles. But, it's possible). The brakes do make noise SOMETIMES.
The sound doesn't seem to affect the performance at all, shifting, braking, idling are all ok.
I was thinking it was due to my leaky radiator cap. So, I replace that. But, the problem is still there.

I've never looked to see if the A/C and heater compartment doors are vacuum driven like on later model cars, but if they are, then you may have disconnected a vacuum line somewhere when you disconnected the A/C.