I can add some more information and deal with some suggestions.
The interference is highly directional and also linked to the RPM somehow. When it initially started it cut in at 1900RPM , after we changed the plugs for new plugs it started at 2100RPM, since the Mags were rebuilt it moved to 2200RPM. As we have totally eliminated anything in the ignition system as the cause of the noise I think it is reasonable to assume that it is linked to the vibration from the engine, the engine has run smoother as the ignition system was modified and improved.
The prop is well balanced and the idea that the prop is somehow out of balance is unlikely.
Yesterday the aircraft had the problem for most of the day whilst it was working, and then I dropped of the last passengers and headed out to the run-up bay to do some testing and the problem was gone. I made 4 attempts over a couple of hours to get the problem to re-appear so that I could do some tests and it never did.
This morning for the first flight of the day the problem came back immediately and was present for every flight during the day. I finished late and did not have a chance to do any testing, tomorrow morning I am planning to do more tests.
The radio guys tested the VSWR of both radios and they were all fine. The other end always gets the transmissions as 5's which would seem to rule out shadowing as the cause. Also there are 2 antennas in different locations on different radio sets that both behave the same way. The only difference being that since we replaced the antenna on com1 with a better larger and higher gain antenna we pick up the interference more strongly with the result that the transmitting station is much harder to understand.
The noise can only be described as a white noise, no distinct pitch or tone, no relation to engine RPM except that it appears and disappears within 50rpm of 2200rpm every time. There is a chop to it, but it does not change with RPMs.
I have run the aircraft with everything except com1 de-energised, including the alternator, with no change.
I think we have ruled out the ignition system as the cause.
We have ruled out the alternator.
We have ruled out every other electrical device.
I have more time tomorrow between flights and I am hoping the problem is still there and I can try some troubleshooting.
Specifically I want to try the radio with the antenna disconnected, with an ungrounded antenna connected, and the hand-held connected to the fixed antenna to try and determine where it might be coming from.
Thanks for all the suggestions so far, I'll keep you updated on my progress.