When the PSU is off there is no AC present the battery is DC it probably is coming from the PSU im not doubting it but my bet is that all of them give off AC noise
Ive just checked the ac noise on our demo panel in the training room and thats 32v, that panel hasnt seen much action at all, mostly used for testing different programming scenarios. It has a couple of wired contacts and a wired detector the old style keypad a GSM/Wifi card and some wireless stuff on it. I am wondering if its a trait of the switch mode PSU