Left channel fading in and out

As I was listening to records last night I noticed the left channel started to fade in and out, not completely, but audibly. No distortion, just slight fade-out for about a second at a random pattern. At first I thought it was the new used record I bought and was listening to for the first time, but it continued when I switched to another record I was familiar with and didn't remember having this issue. What could cause this?

I have a solid state phono preamp, tube line stage, and tube monoblocks. Could it be the speaker? A failing tube? Problem with one of the interconnects? Any input would be greatly appreciated.