Engine rattle at high RPM and high manifold vacuum

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?

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.

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