Burning smell when front defrost is turned on

[aYoung_]

Yesterday morning, when I started the car I noticed this weird smell coming from the vents as the defrost was on. I originally thought I has stepped in poop, (that's what the smell smelt like to me) but my shoes were clean...

I was late for work so I just drove with the smell... The smell lasted the whole commute to and from work (~20mins each way). It was unbearable, I had to leave the window open. After work I looked around in the engine bay.. clean, expect for some dirt, dust & sand.

However after driving on the highway for about 30min and letting it rev until 4-5k, the smell went away by midnight. I recently had an oil change 2 days ago. Was it possible the technician spilled some oil and it was just burning off? The car is usually garaged kept... so I doubt anything died in there. I asked around and one guy said there was bacteria buildup in the vents/AC system? He wasn't very clear...

I haven't checked today if the smell came back.. but I hope it doesn't, it smelled disgusting...