1993 Honda Accord pinging, loss of power, deep exhaust tone

My 1993 Accord started pinging last night out of the blue, I had just pulled onto a bridge and started to accelerate fairly hard in second gear. Suddenly the exhaust got slightly deeper it started pinging like I got bad gas(just filled up with low$ 87 octane same day). Seems to run fine below 1/2 throttle but as soon as I got closer to home and started to go up a hill it pinged terribly and felt like it was running on 3 cylinders. The car has high miles just had timing belt, water pump, etc. Done about 5,000 miles ago. But it has been babied by previous owner, I run full synthetic oil and it has always ran like a vehicle with 1/3 as many miles. I do drive it with a fair amount of zeal admittedly tho. Car has no other mechanical defects. I've owned it for just a year and put 10,000 miles on it and have been extremely pleased and totally amazed at how tight, solid, and maintenance free this car has been. I used to drive Audis/VWs and they were very high maintenance with 1/2 as many miles. My father recently drove this car on a short trip and was totally impressed with it. Which made me happy coming from a guy who drives Lexus's/Mercedes/Audi's high $ new stuff. So I really hope this could be something minor. When the mechanic did the timing belt etc. He said he had problems getting the tensioner set, the belt is very tight and the car whines at idle but his statement was that it would stretch and the noise would go away. Well it hasn't yet, I was concerned about bearings? Could this be related? My first thought was maybe "Knock" sensor or o2 sensor? Just because of the pinging and the apparent increase in exhaust tone. Please advise.