Yes. Trying different sources. CD player. DAC. phono. Cassette. All of them. Get a cassette playing, a CD playing, stream something through your DAC. All at the same time. Keep the volume at 9 o'clock while you switch between the sources. Read the owners manual. Tweak the gain control. Try the switches. I concur it's *probably* something simple.
Please keep us informed. We're all on your side.