I have been wondering what the buzzing was inside of my AC compressor for a while now. The unit has been working fine but it had this annoying and worrying buzzing. Turned out to be a solenoid coil body vibrating. I ended up using a brass shim under the coil body to quiet it down for now. I couldnt find any info out there about this particular issue so I thought I would share this just in case someone else has a similar issue.