Why am I hearing this?

I have a Marantz pm700av av surround amplifier and a Marantz 50-st synthesized stereo tuner. I noticed today the when I had the volume all the way down I could still hear the tuner even though a different input was selected. In fact I could hear this on every input selection except the CD input. The CD input is all by itself on the back panel and all the other inputs are in the same column. Don’t know if this has anything to do with not hearing the tuner when selecting the CD input while hearing it when the other inputs are selected. The volume for the tuner doesn’t increase when you turn up the volume, just the volume for the input selected. Why am I hearing this? What is this called? I bought these two units together for about $1150 new.....wow, 28 years ago! Spending that much that long ago I thought I was purchasing a decent product. Thank you for your consideration and reply.