I have an '89 Toyota around 800 - 1000 rpm under load. Put it in neutral and it goes away. Go over about 1200 rpm and it goes away. One day it wasn't there, then one day it was. It's a noise different than the normal rattle from the valvetrain, lower in frequency, though it's hard to specify exactly where it's coming from.
From inside the car you also hear it briefly when first coming off idle and it goes through that rpm range. The local Toyota dealer says they think it might be rod knock but have to break the engine down to determine for sure. Their assertion was that the reason the noise went away over 1200 is that increased oil pressure stops the rod knock. Does this sound right?
A local engine shop who has a good reputation listened to it, and listened with a stethescope, oddly they found that they could hear it the loudest on the front bolt of the A.C. idler pulley. I removed the pulley to see if that might be the culprit but it's not.
The engine shop didn't think it sounded like rod knock due to the narrow RPM range it occurred in and they disagreed with the idea that higher rpm would make rod knock lessen from increased oil pressure.
I'm in the process of pulling the timing chain cover to see if anything obvious is loose/worn there. In the absence of anything obvious there, any suggestions?
Thanks for all input.