I have a 1996 GMC Sierra with a 5.7 L Vortec. I have spark knock even with NO towing. I have spent a huge amount of money repairing and maintaincing this thing. All new parts either NAPA or factory: tune up, O2 sensors, alternator, battery, power steering pump, oil cooler hoses, replaced all oil seals every where, water pump, radiator, Amsoil engine oil, Amsoil transmission oil, brakes complete engine fuel system flush, K&N air filter, fuel filter, etc, etc... I have not replaced the mass air flow sensor. They said it was fine. I run regular gas, either Shell or Chevron. I've tried a higher octane and it doesn't help. I religiously run Sea Foam in it. I have asked about Dyno-tuning and was told either - don't waste your money or we won't do it on a vehicle this old.
Ok Guys, what's next? I can't afford payments on a new TV.