check engine light came on/i hear a pinging noise from tranny

Hey all, i gota 97 honda civ bout a year ago(10 mos ago) and noticed that something started to ping after like 6 mos. It sounds as if its comin from tranny bu tim not sure. Dunno if this could be it, but i was using low quality gas 87 hess, for about 6 mos. Then i changed to 89 premium hess for the past 4 mos. It still pings. This only happens sometimes, but a lot of the time after i leave up my clutch and press the gas, or when i just leave up the clutch a lil at a time w/o gas. It annoys me a lot. and.....

My other problem is my check engine light just game on. I dont know why, but i just cleaned teh o2 sensor(top one) ah, bout 2 mos ago, changed the plugs, etc. The light stayed off for bout 2 mos, and now its back on. Well, my mom's bf did for me, he's a mechanic, but doesnt know hondas worth a darn. well, help me out please guys, cuz I have the car in the paper and hope to get it fixed FAST....Thanks a lot, well appreciated.