I don't see where it says it does it ONLY under load. I have been trying to get to the bottom of that.
The more I think about this the more I respectfully disagree. All our cars seems to vibrate under load around this rpm. When under load the engine vibration is being transmitted to the wheels and suspension vs not under load when out of gear or clutch engagaged or decellerating. As such I would expect more vibration noise under load and the ac hoses clattering on the inlet tube will make quite a sound.
Did you have to pull off the stainless heat shield to get access to everything? That part seems perfect for rattling around. Lots of nuts and bolts that are hard to get to.
Didn't remove heat shield. Will not clatter when stationary at same revs. Needs to be under load. Cam timing is all good, not a pre-ignition knock. I have the a/c drive belt removed now and am going to put everything back and try it to see if it is the compressor. Thanks again to you all. Keith
Hope you find it. My belts did that on my compressor ... thought my valves were banging around! hope that is it.
Mine sometimes occurred under gentle load, guess some combination of harmonic vibration, motor moving on mounts. Think it happened more when the car was cold too. Cant recall it happening standing still. Anyway, one to eradicate.
It's likely not the problem but I was thinking about the two hoses running from the compressor that go into the bulkhead on the right hand side. With the compressor mounted to the engine there is vibration while the engine is running whether the compressor is running or not. On my car that vibration at a certain rev range caused those hoses to resonate against some sheet metal causing a noise. Using a piece of sliced radiator hose I tried to isolate the hoses from metal and join them together as they crossed the chassis member, hopefull reducing their movement. This worked and stopped the vibration at similar revs that you mentioned underway. Again, this probably isn't the clatter you are hearing but is an easy one to rule out.