Knocking at low RPM

97 pontiac transport, 145,000 miles, 3.4L. There is a loud knocking at low rpm. When you rev above 3000rpm, it smoothes out. We have checked the plugs, wires, distributor, coil pack, computer, ignition module, starter relay. Everybody I talk to has a different diagnosis-even mechanics. I am thinking we are either on a thrown rod or stuck valve. Any other ideas? I am trying the small things first before I replace the whole motor. About a year and a half before this problem started, we sent it to the shop to repair a blown head gasket. This problem started in a instant, it was not a slow progression.