AC Hum Mystery

As I was leaving my garage I noticed an AC hum. Looked around, no radios were left on. I was puzzled until I remembered that mounted down in the front corner on a switch box there is a plugin relay. This is part of the remote control circuit for the lights mounted under the garage eave. The problem is that I used DC relays because I had a surplus of them. Each of several relays has its rectifier-filter and the filter for this hummer was simply failing. Replaced it and all's well. I realize that AC relays are preferable, but I did not want to spend when I had all the free materials at hand, and this trouble was the first one in over 30 years. Back when I started installing lighting controls and various gadgets around the house I made sure it all was documented, i.e., wiring diagrams, parts lists, etc. Essentially what I have is a service manual on my house and it's proven to be an asset.