Check Engine and strange sound/smell

Honda, Accord, 1991, 272 220, 2.2L F22A4, Automatic

Hi,

Once and a while the check engine light will turn on. Not frequentally but sometimes. I just shut the switch and turn it on again and it will disappear. But it still cought my attention. My friend says that it could be the o2 sensor that's making it turn on beacause it's not the original muffler that's on it.

After, when i'm riding along, I will frequentally hear this strange sound coming from under my car. Like when a you drive over a branch in the road. The exact same sound. And no it's not that :p. Mybe it's just a loose piece of plastic?

Finally, sometimes when i'm riding at 100-120km/h this odor of something overheading smells in the car. I'm the only one that can smell it. Am I crazy? It think it comes trought the air vent. I did spill some oil on the motor head while changing the oil about a month a go. The stain is still there. Could it just be the oil cooking?

Thanks for your time.