Smell for air con/heating

Our Ulysse 2.0 16V Eleganza (2003) has a strange smell coming from the heating system. I thought it was the air conditioning causing it, so I used one of those air conditioning cleaning bombs, which you put in the back of the car with the air con on full for 10 minutes. The strange thing is it makes no smell when the air conditioning is on cooling, but if you increase the temperature and the heating kicks in it smells like an old and musty sock !!!!.
Should I used another air con bomb and leave it on heating rather than cooling to clean out this part of the system. I am not sure how the climate system works, I guess that when the system in cooling the air doesn't get anywhere near the heater matrix so that's why the cleaner didn't work.

Thanks & any ideas

have you changed the pollen filter? usually the reason behind smelly air....

Thanks for the tip, it is about 8 months old so could probably do with a change (mind you it's almost and engine out job to change it) but what I don't understand is that it does it only on the heating cycle. I have done about 1,000 miles in it since I noticed the smell and it hasn't lost any coolant so I don't think it is a leak from the heater matrix.

I always thought these smells were caused the evaporator on the air con.

I don't know about the Ulysse climate system specifically, but normally the 'Maximum Defrost' function (assuming you have this) sets the following:

1. Fresh air intake (not recirculated),
2. Maximum blower speed,
3. Maximum A/C,
4. Maximum heat, and
5. Demist air vents.

This will direct fresh air through the air conditioning for maximum cooling (to condense any moisture out) and then through the heater. This means the air directed at the screen is both dry and warm, to optimise the demisting effect.
I would guess that this would be the best setting for using your cleaning bomb, although the face and floor air vent ducts will not see any air flow, and neither will the duct for 'recirculated air'.

Where do you put the 'cleaning bomb', though. If it's in the car do you need to select 'recirculated air' ?