S60 - smoke coming from bonnet!

[Meja]

First excuse me please for my poor English! But I really need very much some advices. Here is my problem, I got S60 2 years ago and it's happened few time last year and now again, only in winter season, when heating used. It's never happened in summer, whenever air-conditioning was ON.
So the problem is, that sometime happens smoke comes from the bonnet! I think it's can be OK, when I start car in the morning, after cold night turning ON venitlator and heating on full volume, then I see some smoke come from the right side under bonnet. But strange thing is, that it's doesn't happening every time, sometimes yes, but sometimes I do same thing in same circumstances and nothing coming from the bonnet.
What worry me most, when it's happens like today, it's pretty warm outside +5C, I started car and drove about 6 km, using heating and ventilator ON just a little(not full volume at all!) and when I stoped on the red light for like about couple of minutes suddenly a lot of smoke coming out from my bonnet! Smoke has white color and kind of burning smell. I switched OFF immediatly heating and ventilator at all , smoke dissapeared. Same things happened few time last winter and it's doesn't happens everytime, even in colder days and more wet days and even when I use heating more, then today.
When I was for technical inspection, I mentioned the problem, but when they tried to have heating ON for a while, ofcourse nothing happened, that's why they couldn't recognize the problem. But it's really worry me, you know it's not very pleasant to seat in the car with coming smoke under your bonnet and wondering is it burning, or not.
Please, I will be very thankful for any advices and your experience!