I'm told it may be a cracked flywheel (flex plate). Here's my symptoms. I have a 1991 S10 Blazer 4x4, automatic, 4.3L. 125,000 miles. Runs good. About a week ago I noticed a knocking noise after shifting into gear one cold morning. The knocking seems to go away after I start driving but I will hear it once in a while while driving but not really loud. It seems less loud in warm weather and much louder on a cold morning. Now, if I let the truck warm up and idle in the morning you will not hear the noise until I shift into gear and then it starts. The noise speeds up if I rev the motor. It will make the noise in park too but not as loud and, again, goes away when driving for the most part. Any ideas?

Loose torque conv bolts?