1995 Chevy

1995 Z-71, 350 w/TBI, 5-Speed Man. Trans. with 149,000 miles.

Engine and trans. were rebuilt at approx. 120k miles. There is a constant whirring noise at all power settings, meaning at idle, acceleration, under load, etc., but on deceleration, with foot off the gas, in any gear, at approx 1500-1200 RPM, whirring noise disappears until I push the brake or clutch. Also, while driving, in all gears, during normal acceleration, I can hear a slight ping, but when I push the gas down hard, after 2-5 seconds, hard, loud ping can be heard, then goes away. Once the ping goes away, the Service Eng Light comes on. If I mash the gas again, the ping is not there. When i shut the truck off and then start it again, the Eng. Light is off. I have it diagnosed and was told twice itwas the o2 sensor. They were replaced, but the same thing will happen. Was also told it was spark knock and that I should use 92 octane gas. I have been using 92 since eng. was rebuilt since it was recommended by the mechanic who did the work. Have not done anything else to it, and my guess is it may be the timing. If anyone has a suggestion, I would appreciate it.