I have a 99 ford expedition. Turn the key and one loud clunk?

I have a 99 ford expedition… Turn the key and one loud clunk?

I have replaced the battery, 2 starters, terminals are clean, turned the engine flywheel to make sure it is not seized but when i try to start it, the starter engages and it just seems the starter doesn't have enuff to turn the flywheel. It has a new battery but under load it won't turn it over. I have also bypassed the solenoid on the firewall at it does the same thing… Clunk!