Oil Burner Forced Hot Air burning smell

[wdoa]

For the last several months about 50% of the time when the burner turns on for the first time in the morning (it is usually off for several hours during the overnight) I get a nasty burning rubber kind of stink. I can cycle the burner off and on and everything then works okay (i.e. get heat no funny smell).
When I cycle it I just turn the thermostat down for a few seconds, the blower continues to blow but the heating portion is off.
I've had 2 different technicians out and they have checked it out and can't find anything wrong. It almost seems to me that there is something accumulating on the fuel nozzle after being off for a few hours. The smell doesn't resurface if the thermostat is kept at a high enough temperature that the heat kicks frequently. The smell only occurs if the the furnace has been off for several hours. Any ideas? BTW I do have CO detectors in the house and they have never tripped when the furnace is in "stinky mode".

[HayZee518]

the smell shouldn't be coming from the heat exchanger because it is a closed receptacle. what does come to mind is a slipping belt if your fan is remotely located from the motor. the fan is mounted in rubber bearing retainers on the "spider" - these may have shrunk and are spinning inside the spider. once the fan heats up the bushings will have expanded and not slip.