220K 1998 c70

I've been loosing engine coolant from my system at a variable pace.

It vanishes at a variable pace but I find myself needing to top off the expansion tank after a few days.

Replaced the heater hoses, receiver block, seals and tubes to the heater core. Fluid is doesn't seem to be leaking on the inside, and I'm positive I'm not losing it from the received block at the firewall. I can't find a detectable leak at the heater core either. Main hoses and the expansion hose are all good. Generally, loss rate is slow, unless I've got the AC on.

I jacked up the car (with it running) and found that my coolant was leaking out from around the metal heatsheild on the bottom of the car. With the AC on, I can see it dripping out at an alarmingly rapid pace. It's coming from somewhere above (I can divert it slightly with my finger, but I'm not sure where the leak is at. It seems to be dripping more on the passenger side of the tunnel, but I'm losing it on both sides of the Cat.

Turning off the AC makes it stop immediately. 'though my coolant is still going somewhere even with the AC off.

Is there a known trouble spot somewhere above the Cat? I can actually smell the coolant in cabin air (with the fan on), but as I said, I can't see or feel any leak at the heater core junction.

I'm missing something big.

Any and all suggestions are appreciated.