Have this weird issue with my 88 gta. I bought car with a blown up motor, swapped in a motor that my old auto teacher had, ran it for about 500km before it spun a bearing (shitty rebuild). But before it did, I could hear this rythmic knock/rattle when at 500-1000rpm in gear or park. Pretty easy to hear it in the car, but really hard to hear when out of the car.

I assumed ok, but as probably the **** motor rattling before it kersploded. Swap in another motor (known good running one, got to hear and see it run. Made 0 noise. Came with no accessories. Put my accessories on it, notice same noise is present with this motor. So that leaves the crank pulley itself, not the balancer, because I can see the pulley is bent and it wobbled on the old engine as well as this one, so I picked up one of those. Could the transmission be making the sound? It shifts hard and good. Doesn’t slip at all. But I’m ruling out the engine.

 

check transmission mount
the rubber block under it and corresponding Crossmember
if there is anything loose or broken it could be at an specifing resonance vibration gets feed into chassis and these noises are most likely only heard inside the car

also motor mounts

 

I have checked the tranny mount…and the motor mounts are poly

 

Easy enough to see if it's coming from the trans.

Unbolt the converter from the flex plate and push it back out of the way, and start up the motor.