Trace for a low buzz in one channel on AE3 preamp?

I have Audio Electronic Supply AE-3 Preamp. The previous owner replaced diode rectifiers with a rectifer tube.

I notice that the left channel has a slight buzz (hum). The buzz is consistant, regardless of the position of the volume on the preamp. As my amp has volume as well the buzz comes out only at the max volume on the amp (max. gain). The buzz is not that loud, on my 92 dB speaker, it can be heard only within a few feet. Don't hear it at the listening seat, unless really pay attention to hear it. The buzz exists with all other amps I have tried as well.

I open the pre and look for loosen soldering points, but don't see any. As it happens only on one channel, I think it's should be in the gaining circuit on that channel and don't think it's power supply circuit as both channel share the same power supply.

Where/how should I trace it for this problem?

Thanks a lot,

Ake