I'm pulling my hair out over this. After replacement of all top-end engine parts, cam break-in, and roughly an hour of running, my 305 HO has developed a knock near the rear of the passenger bank that comes and goes seemingly at random. One minute it'll sound like the engine is going to explode, and the next it fades back to nothing. I've jammed a stethoscope all over the thing when it happens, and it all just sounds like regular valvetrain/lower end noise (I.E. whirring/clicking, with no real knocking at all). It happens in time with cam rotation, so I suspected valvetrain, but after taking off the passenger side valve cover and turning the engine by hand everything looks completely fine. All the valves move, nothing is rubbing, there's no metal flakes ANYWHERE, and I drained the oil just to be sure. The engine runs great and doesn't change RPM/power when this happens or when it goes away, oil pressure is a steady 30psi at idle and 35-40 under regular driving.

If it didn't sound like someone was banging on the firewall with a hammer when it happens, I'd ignore it, but I'm trying to sell the car soon and I don't think buyers will be willing to pay nearly as much if it sounds like the engine isn't going to last more than two minutes, regardless of whether or not it's actually healthy. I've checked the harmonic balancer as well; it's fine. It's not new by any means, but the rubber is still nice and firm and it doesn't rotate at all without turning the crank. Fuel pump makes a tiny little clicking, but honestly sounds perfectly normal to me. The worst part is that this sound is barely audible in the engine bay, but is apocalyptically loud inside the car. I suspected the exhaust was hitting something, but the noise happens exactly in line with cam timing and doesn't change volume at random - it slowly fades in over five or six seconds, stays a while, and then fades back out.

Lemme know what you think. It's driving me insane.

 

FINALLY GOT IT. It was spark knocking on cylinder 6 only - I'd hooked up a vacuum line to the port in the cylinder 6 intake runner that was previously hooked up, because it hadn't caused issues before. I guess sometime during the top end rebuild I'd removed a line from that vacuum circuit, so it was just dumping air into 6. Also fixed the weird idle and general rough running.