Heater blower slowed down and sulphur smell

[Rorcus]

Just popped out on a short run this afternoon and turned the blower to 5 set on cold (non functional AC at the mo). We were sitting behind another car and noticed a strong sulphur smell, at first I assumed the car in front had a bad CAT but when I got back noticed the blower was turning very slow and would not adjust speed from 1 to 5 this and the sulphur smell was coming from the Vent.

Burned motor? or does mine have a "hedgehog" regulator that might have burned?

 

[Rorcus]

hmm.. update.. (can't edit the above post)

When I got in her this evening after work the blower now blows full speed again.

So I'm possibly thinking because today was reasonably hot and I had the blower on cold but my heater duo valve has a snapped valve plunger it may be letting hot water in to the matrix which in turn is causing the regulator to slow the motor down on purpose to stop it filling the car with hot air when set to cold (based around ambient temp outside being hot for a change?).

Then the sulphur smell could a combination of bacteria in the AC evap/cabin filter and the first time I've used the blower at full speed in the hot.

Plausible?