Noisy Furnace

My Carrier Hi-Efficiency furnace exhaust has a strange odor. I noticed this about one week ago. It smells like the natural gas is not completely burning. However, furnace operations is normal and house is heating ok. No odor in the living area, although there I can detect a faint odor when I opended the front of the furnace. Flames appear blue/normal -- no yellow. Any suggestions that you have for what I might be able to check to diagnose this problem?

I have an oil burning Lennox D1202-70-3 (I believe that's the model number, its a little worn). It has a Wayne MSR pump. My problem is, the furnace was cleaned this time last year. That was prompted by a similar problem. The fan stayed on constantly, signaling a problem. I tried pushing the reset button, but that did not work. When the tech came out, he replaced 2 filters, and the nozzles, and readjusted the electrodes. Unfortunatly, I had a tech out a 2nd time, since the nozzle immediately plugged. Anyway, I'm experiencing a similar problem now. I set the temp, and the heat comes on, sometimes. Other times, the house gets cold, I press the reset button, it takes a bit for ignition, I get the black smoke and rumbling, and I have fire. The flame looks normal. The furnace may run fine for a couple days, then not start, and I press the reset button, and it comes back on for another couple days. I've had to hit the reset 2x now in the past week. Unfortunatly, I do not know much about this unit, since I don't have an owners manual. I purchased this house just about 2 years ago, and it is now 20yrs old. I have no idea how old the furnace is. The tech that came out last time was from/recommended by my oil supplier. Is there something else I can do myself for troubleshooting? Or should I just call the tech again?