What can happen is that a battery chargers output is actually dirty DC, good enough for charging a battery, but as it's not fully rectified it can look like half wave AC to anything worried about frequency.
An ignition coil works by charging the primary side (points closed) and when the points open the field in the primary side of the coil collapses (as it's no longer charging up with the points open) as this field collapses it energises the secondary field on the ht coil, which then collapses once the primary is 'empty' causing the spark - this is happening 60 times/second (for those in the USA), 50/sec here the UK when connected to a battery charger with poor rectification.
Get a well charged battery on there and try it, it should stop the buzzing without the battery charger on.
If you have any switches on the charger, play with them and note the buzzing noise of the spark will change as the chargers output changes, another test is that the buzzing will seem worse after you try cranking on the starter as this drains the battery, so the charger tries harder to re-charge it.
Sorry for the science bit, but I like to give full answers so that people can see the 'why' behind it