think the engine is a gonnar

[kris's66lwb]

so driving my truck yeaster day and it was all normal till a light then she started making a really bad rattle noise and ran really poor took her home puled the plugs they look normal pulled the cap and rotor alil dirty cleaned em still no change took off the valve covers and checked all the rockers all is good there too still the nose contenues im thinking i lost a ring and have no compression at idle no vac leaks that i can detect at all it runs great at high rpm any help guys i cant have this truck die on me while im still building my transmission for my car