Engine rattle at high RPM and high manifold vacuum

[woofhaven]

I have a '73 Midget with 1275 cc engine. Engine was rebuilt about 8,000 miles ago, and valves / valve seats replaced for unleaded use. Ignition switched over to electronic, but still uses distributor (new) with vacuum advance. All original emission controls are still in place -- including the air pump.

At about 4,000 rpm, the engine lets out one heck of a rattle. I can't tell where it is coming from. The wierd thing about this is that the rattle appears at slightly lower (say, 3500) rpm if I am not under power. In other words, high manifold vacuum seems to be related to the noise.

This noise has been present since the rebuild, so it has survived several tune-ups, ignition adjustments, etc.

Any ideas what this is or whether it is serious?

[AccordCodger]

4000 rpm is a bit low, but if the valve springs are weak (or they were the wrong ones at rebuild) it sounds like valve bounce to me.

[woofhaven]

Thanks. What is valve bounce? Would there be any other symptoms?