1989 Chevy K1500 TBI-383 knock sound only when outside air temp is cold

I have a problem will a new 383 I built that started after having 18K miles on this engine. The problem is a knock sound that only happens when the temperature outside is cold. For example, if it's in the morning and the temp is below 45'F for example, I have a knock that goes away after one hour of freeway driving. In the afternoon or if the morning temps is above 55'F I have no knock sound at all. Some people I talked to say I have a piston hitting the head when the engine block is cold. Before this knocking sound I had a sound coming from the engine that sound like a turbine-wheel winding up will RPM. No, the sound wasn't coming from anything part of the engine driven by the fan belt. The noise is gone, but now I have this knocking sound. Some say I may have spun a rod bearing. Any thoughts? Byron T Tucson AZ