Topband: 9 kHz heterodyne BCI

[Bob Kupps via Topband]

Hi we just got QRV on 160 right before the SP and noticed AM BCI at 1800 kHz and every 9 kHZ  up from there. Since 9 kHz is the BC channel spacing here I suspect two adjacent stations are mixing. Somehow somewhere. Maybe here in my station, one of the BC stations or somewhere else?
Unfortunately our new HiZ 4/8 array was rendered inop before we could finish it due to rodents chewing away sections of the feedlines. We rigged up 2 2-way one wave Beverages and these heterodynes were the same strength from all 4 directions on them. Opening our tx vertical had no effect.
Anyone know the likely cause/cure for these observations?
TIA 73 Bob HS0ZIA