HP7640 making humming sound while sleeping

Question

My HP 7640 is working fine. but while asleep it makes a high pitched, low volume, whinny humming sound.  If I touch the NFC icon or do anything to wake it, the sound stops.  

Answer 1

Had the same issue, went to store, changed the printer for same model... same issue.
the "consumer" options only let you prevent the printer from going to sleep for up to 15 minutes, you can't prevent it forever in that menu.
Called HP support, asked for escalation to level 3 support to request a way to avoid it going into stand-by mode. Yes, there is one but you have to get into the engineering menu of the printer to be able to do that. Here is the step by step that Level 3 support walked me trough, they worked for me. YMMV, so do it at your own risk:
1) get to the enginnering menu on the printer
Read this first before pushing any buttons: on the top left corner, NEXT TO THE SCREEN, there is a home button, some times it is lit up sometimes it is not, on te bottom left corner, NEXT TO THE SCREEN, there is a back arrow button, sometimes it is lit up, and sometimes it is not (THESE BUTTONS ARE RIGHT NEXT TO THE SCREEN, NOT ON THE SCREEN)
Now push as follows (even if the buttons are not lit up): BOTTOM LEFT BUTTON ONCE, then, TOP LEFT BUTTON 3 TIMES.
The screen should now show the Engineering menu.
2)how to get to the menu where you change the system mode timeout settings:
Look for a menu option called "Underware".
(Stop giggling and send a telepathic mental high-five to the HP engineer who named that menu)
Go into "underware".
several new arrows appear. Click the UP arrow until you see "System Menu"
Hit OK
Click the up arrow until you see "System Mode Time Out"
Hit OK
3)BE CAREFUL NEXT - dont go past zero - AND BE PATIENT.
you now see a large number, like 800 or something, push the down arrow once. you are now at 799, Repeat again and again until you reach zero.
(Stop being angry and as you keep pushing 798 times do send a telepathic mental -duh- to the HP engineer who built this UI menu.... on a touch screen... without adding a number pad)
DO NOT GO PAST ZERO!  (I dont know what happens if you do)
Once you see ZERO, HIT OK
You now see "System mode timeout" but not the zero
4)Exit back
Push the red X
Push Home.
Your printer will never go to sleep. Enjoy your super quiet home-office, without a high-pitch whine from the printer, while HP troubleshoots what's going on with the standby power modes in this series and offers a fix.  You did register your printer purchase right? otherwise how will you find out?
D-user.