Has anyone had a knocking coming from there facet silver top pump. I doesn't do it all the time but when it does, you can hear it over the exhaust noise.

By knocking do you mean a different noise than the usual clatter when they are drawing fuel.

Yes it's different, it sounds like a hammering on the chassis

Mine does the same when its priming. I think the reason is that its bolted directly to the chassis when it should probably be mounted on a damper of some sort.

A couple of rubber bobbins between the pump and the chassis is the norm, won't stop the noise but makes it more bearable.

If they run dry due to fuel surge in the tank when going round a bend the clacking noise changes till fuel gets back to the pickup.

I'd agree with that, when it is struggling to get fuel to it, ie tank is pretty low and it is managing to get some air too, then they clank as such. Mine was on bobbins and mounted to fibre glass and I could still hear it at times over the exhaust..

I did have one fail and I don't recall the noise getting worse before it went, it just went and blew the fuse, so at a guess I wouldn't say failure is imminent.