So I have a 95 Bonneville SSE with 430,000 kms that started making a strange rattle/ slapping sound. As soon as you fire the engine up the sound starts but seems to quiet down if it is turning at over 1500 rpm ro if you are going over 40 km/hr.

I was leaning towards the timing chain because of how many kms on the eninge and because when the A/C compressor would kick on and put more stress on the drive belt the slapping sound would get louder. I tore the front of the engine down to find that the timing chain is not the problem. I then tore the engine almost entirely down to find it is not the valves, valve springs, bottom end, or balance shaft.

I'm now at a complete loss as to what could be causing the sound and am seeking help.

Did you ever happen to get a video or sound clip of it?

Unfortunately not

You checked all the accessories, lifters, rods, rockers etc?

Yeah, they're all good... turns out it'* a cracked drive plate. Thanks for the help.

Drive plate is another name for flywheel/flexplate?

Yeah that'* the same piece