On the N55, the most common coolant leak spots are:
1. Expansion tank - develops hairline cracks
2. Electric water pump - the connector area
3. Thermostat housing - the plastic housing warps
4. Heater hoses at the firewall - hard to see
Since you smell it but cant see it, my bet is the expansion tank. The cracks are tiny and the coolant evaporates before it drips. Use a UV dye kit to find it.
Pressure test the cooling system. Rent or buy a cooling system pressure tester (about 50 EUR). Pump it up to 1.5 bar and watch for pressure drop. Then inspect with a flashlight.
Also check the charge pipe to intercooler connection. On the N55 the charge pipe can crack and if coolant is leaking near the turbo it evaporates instantly - you smell it but never see it.
had this exact symptom on my N55. turned out to be the electric water pump weeping from the electrical connector. very small leak that only happened when hot. replaced the pump and smell went away.
the pump is about 250 EUR aftermarket and maybe 2 hours labor.
