Portastudio 246 Channel Hum ???

[arcaxis]

Just recently while recording with the 246 I discovered that track 4 while paused - heads engaged to the tape (not in record mode) had a hum in the background that the other tracks didn't have. This was with the monitor gains and monitor master at almost full gain. Tracks 1,2, and 3 had some background hiss, but no hum. I set about unplugging all connected effects and preamps thinking there was a ground loop some how. This didn't get rid of it. I figured at this point maybe a component had gone bad that was the cause of the hum.
I hadn't cleaned the heads or degaussed for a while so I did this. Now strangely track 4 no longer has the hum. Could degaussing the heads have gotten rid of the hum ??? This one baffles me and I'm wondering if anyone else has run across this. Last I knew degaussing of the heads got rid of residual magnetism that could cause high frequency loss in tapes.

[jake-owa]

I guess the magnetic buildup could cause a hum if it was strong enough.