Hey guys i have a 1991 camaro rs 5.0 when im driving at a steady speed (40 or so) rpms low and tranny is in a high gear and i press on the gas, right before it downshifts to a lower rpm it makes a rattle/knock noise then the SES light comes on throwing the code 43 which is knock sensor, ESC module. It doesnt have a tick or knock any other time from start up to driving except on downshift can you guys tell me what it could be?

 

One of two things, either your bearings have too much clearance and the knock sensor thinks its pinging and is trying to fix the knock by adjusting fuel and timing. Which of course would have no effect.
or

Option two is for some reason the engine leans out right at the point you described so bad the computer can not compensate enough to stop the pinging.

What I call pinging = detonation

I have seen both situation, back when your car was new if I got a work order with you description I would assume the car needed a crank kit until I proved it didn't. I put a lot of crank kits in 5.0 under warranty. In most cases when lean is the issue you get a lean code with code 43.

To a non-mechanic the two failure modes sound similar but to a pro pinging and bearing knock sound nothing alike. The only way I can explain it is to say bearing knock is rhythmic, predictable where spark knock sound chaotic, marbles in a blender kind of noise.

Edit/after thought
A high miles motor could build up enough carbon that is raised the compression high enough to cause pinging. This would show up at lower RPMs under load because cylinder pressure is high. Not something I ever saw under warranty but possible.

You could run some 93 octane fuel in the car or race fuel additive, if the pinging goes away or gets a lot less then its not a bearing issue. If it is bearing noise and you catch it before the bearing spin you may be able to resize the rods and get everything back to spec, if a bearing spins the repair cost can skyrocket fast.