The gurgling is coming from the heater valve cycling, and can be normal in some cars at some ambient air temperatures. You are hearing the increased flow of coolant to the heater core as you come off idle, as your auxiliary pump is non-functional.
Smells inside the cabin are most often the result of something happening under the hood, not in the passenger compartment. Bear in mind that cabin air comes from just behind the hood, and particluarily on older cars the smells tend to escape the engine compartment to be drawn into the cabin inlet.
You said your auxiliary pump was leaking. I would replace it first. Look for any other coolant seepage under the hood. Then drive the car and see if the smell is still there. Chances are, if your heater core was leaking, you would get a film of coolant on the inside of your windshield after running your defrost for a while. They usually announce themselves rather obviously. You could also remove enough of the under dash and console to try to view the hose connections to the heater core. They could be seeping slightly.

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'96 C280 (gone)
'97 C36
'05 C230k