My engine makes a hollow knocking noise on cold start and when shifting into gear. It lasts about 5-30 seconds depending on how long the car has sat, and about a second when put it into drive or reverse. If I pull the fuel injector fuses and crank the engine till oil pressure build up to 60PSI, then start the engine, it still makes the noise but for less than a second and its about as faint as the tail end of the mp3. sounds like its coming from the lower back end of the motor, but its hard to tell. The engine also hesitates, but not consistently and usually only when the coolant temp is below 100F.
The car is a 1985 Trans Am (305 TPI, Automatic Trans)
The lifters seem fine, valve adjustment seems fine, timing is good (was at 8 Adv. when I bought it, set it back down to 6 Adv.), oil pressure is constant 60PSI (dips to 40PSI only at 160F but immediately goes back up when it gets hotter or colder), RPM does fluctuate about 15-20 RPM at idle in park, Fuel system is good, and it runs fine when driving. I have a feeling it may be temperature related since it started happening right around the time temperatures would get down to 50F at night and got worse as it got colder.

Any idea what could be causing this?

 

I have heard a similar noise when a cat converter core starts to get loose in the housing.

 

That definitely sounds exhaust related. Like a banging heatshield or as Vader said the Cat is coming apart.

 

Thanks Vader and Edge, I'll check the cat this weekend. Hopefully that's it.