Have a '78 280z, all stock. The issue is this, when sitting still , I can rev the car to 3-4k with no issues or unusual sounds. However, when driving under load, if the RPM gets over about 3200 during acceleration, I get what sounds like either detonation or some type of valvetrain noise. It's very loud, and I am afraid to rev beyond this limit for fear of damage. Setting valve lash does no good, and it seems to have very little to do with timing, as I can increase or decrease timing with basically no effect. I am open to suggestions as to what it may be. Thanks in advance for any help you can give.

Usually it is the heat shield under the intake.

Rev the motor by hand slowly until you hit the rpm where the part vibrates

Use your ears to find where the noise is coming from

Grab the noise part with your gloved hand if safe to do (nothing will burn or grab your hand/sleeve).

I will check it out, it could just be the right harmonic under load to buzz the sheetmetal. Like I said, I can rev it to 4k sitting in the barn....no noise, and I'm reluctant to rev it much higher with it just sitting still. Thanks again, I will give it a look.