Smoke coming from a/c vents on ford expedition with sweet smell

The problem is the heater core leaking behind the dashboard. Heater Core Removal & Installation 1997-2002 To Remove:
NOTE: Heater core must be vacuum leak tested before it is remove from vehicle if a heater core leak is suspected.

1. Drain engine coolant.
2. Remove instrument panel.
3. For 5.4L (4V), if quipped, remove junction splash shield.
4. For 5.4L (4V), if quipped, disconnect starter relay cable.
   
   
5. For 5.4L (4V), if quipped, remove junction block bracket.
6. Clamp inlet and outlet heater hoses.
7. Disconnect heater hose from heater core by gently pressing it toward heater core.
8. Remove A/C plenum demister adapter.
9. Disconnect vacuum line.
10. Remove 13 plenum chamber top screws.
11. Remove blend door assembly from case.
12. Remove heater core.

1. Install heater core.
2. Install blend door assembly to case.
3. Install 13 plenum chamber top screws.
4. Connect vacuum line.
5. Install A/C plenum demister adapter.
6. Connect heater hose to heater core.
7. Release all clamps from inlet and outlet heater hoses.
8. For 5.4L (4V), if quipped, install junction block bracket.
9. For 5.4L (4V), if quipped, connect starter relay cable.
10. For 5.4L (4V), if quipped, install junction splash shield.
11. Install instrument panel.
12. Refill engine coolant.

Sounds like your heater core is leaking, You could do a cooling system pressure test and check the a/c drain under the hood for coolant leak, sometimes it will also leak on floor on passenger side.