So my story goes like this..
I bought a 1967 cutlass supreme with a 1970 Oldsmobile 350 and turbo 400 transmission.. I drove that car everywhere, no problems ever. No overheating oil pressure always great. I had transmission problems so I swapped to another turbo 400 transmission. I kept the same torque converter. About a month later, I started hearing an engine rattle . Oil pressure was never affected, but I did not drive it very far once the rattle was detected. I was going to pull the motor apart and inspect the bottom end. Instead, I decided to install a 455 in place of the 350. The motor originally came out of a 1970 oldsmobile Delta 88. I drove the car a few times with no problems whatsoever. All sounded great and quiet. Anyway, after installing the 455, I have the same engine rattle. My question is what could be causing the rattle? To me, it sounds like a Rod knock. It idles great, but the rattle begins at around 1500 RPM, it seems to quiet down a little around 2500 RPM, but I have not reved it any higher because I do not want to do any more damage if there is something wrong. It is the exact same as it was with the 350. The only parts I used off the other motor, was the distributor, the valve covers, the fuel pump and the starter. Could a bad torque converter or transmission cause a rattle like that? I did notice when installing the torque converter back into the transmission, after it was seated on the shaft, there was movement up-and-down with the torque converter. Any help or suggestions is definitely appreciated. Thank you.

 

I would check the torque converter bolts, loose or wrong size, or came loose and elongated the holes. Easy tom check. Flex plate could maybe loose or cracked also.

 

Chances that a noise would transfer from one engine to another is impossible. So, you need to check elsewhere, accy drive pulleys, torque convertor, would be a good place to start.

 

Hi 67, the more info you can provide when asking for help will eliminate a lot of typing on our end.

Need a better description of this rattle and how it's induced—also, more info on the spark and fuel system....carb type, points or HEI distributor.
When does it make the noise...at idle in park or when accelerating under load?
Are you new to Oldsmobile and automotive servicing?

Right off the bat, I see commonality as the 350 & 455 used the same distributor.
Are you sure it's not spark knock you are hearing due to an overly aggressive base timing setting and distributor advance curve? Are you using the vacuum advance? If yes, where is the vacuum line connected?

Rods will make a deep kuthunk kathunk at idle and will pick up cadence as the engine RPMs increase. Generally, a loss of oil PSI accompanies bad bearing clearances.
Upper-end, cam lifters exhaust leaks will have a higher-pitched clatter with a quicker cadence.

To eliminate the torque converter and trans from the equation, unbolt the converter, push it back at the trans so it's out of the way of the flexplate. Start the engine. Sound gone or not.
Did you inspect the flexplate for cracks?

 

Starter bendix is returning (fully retracted) and holding securely?

 

Reusing a converter from a junk transmission with a good one (or rebuilt) is a huge mistake. Whatever shrapnel what was in the bad transmission just got pumped into the “good” one. Especially if the converter itself was the failure point.

Now, first thing I would do is remove the oil filter and cut it open. If the filter media is clear of debris, you can be reasonably sure the engine isn’t the cause of the noise. That leaves belt driven accessories, flexplate, or other external parts.