I find it to be a few things:
- weak coil (hold coil or coin lock (on the door)) - normally replace
- spacing- sometimes the latch doesn't hit the relay flat. I've done the packing tape method, but that will also make the coil weaker.

What I have found that when I have a really bad buzz that the longer switches are adjusted (away) from the coil and is making it difficult for that relay to hold the latch tight and therefore it buzzes. So if I can I will create less tension and move the switches slightly closer to the latch so the coil doesn't have to work as hard. So in short...adjust your switches to create less tension on the coil.

Make any sense?