Just guessing here:
Since you only smell the coolant inside and don't get a puddle of it in the passenger footwell, I would say it's not the heater core.

Low speed the coolant is being burned through a small crack in the head gasket, head or block.

High speed causes said crack to open more and force exhaust into the cooling system causing it to over pressurize and burp it out.

You should get a leak down test or radiator sniffer test to rule out any cracks.