99 ford ranger 3.0 weird hearing issue?

I have a 99 ford ranger 3.0. Ok let me try to explain this. I m puzzled on this one. I put a thermostat in the truck and bleed the cooling system. The truck is not over heating or steaming. No white smoke, no bubbles in radiator or resivoir. The temp hand will go up to where it s supposed to be. And when the thermostat opens it goes down. Then back to normal in about 40 seconds. And don t say it s the sensor. Bc I hooked up a manual gage and it done the same thing. I m lost this is the 3rd stat I ve put in it. I can smell antifreeze ever once in a while. But can find 0 leaks. It does have a slight miss on cylinder 6. But I pulled all the plugs. None are wet or show any sign of coolant. Oil is crystal clear.

Read more: 99 ford ranger 3.0 weird hearing issue?