Sonicare hx9642/20 toothbrush making short humming noises and will tend to turn on on its own. How can I fix this?

There is probably no fix for it, as it is not made to be repaired; if it malfunctions, you are supposed to discard it and get a new one.
However, it is likely the problem is in the switch of the toothbrush.
It may have some foreign matter inside that is causing the contacts to close when not in use, and that its why it is turning on all by itself. If you want to try something to fix it, I would get a tank of compressed air and a blower and try blowing the air into and around the off/on switch.
It might fix it, or it might destroy it.

Hi Noel:
The instructions that came with it should tell you.
I'd guess at 2 possibilities:
1) Needs re-charging
2) Brush needs replacing

Are you sure the brush attachment is "tight" to the handle? Do you have a new brush to try? It should confirm handle is bad or it is the brush attachment "tightness" .

maybe the battery

Probably two reasons for this. 1) the contacts at the base of the brush are not clean which will prevent proper charging , the low battery beep is telling you to recharge.2) the battery needs replacing.

Turn it on and put it on the charger while still on. This should turn off toothbrush. And it stopped the blinking!

The best tutorial for repairing Sonicare toothbrushes is here:
Repairing your Sonicare
The problem is either a corroded circuit board or worn-out Ni-Cad batteries. Both repair solutions require soldering, but full repair is possible.

I had mine for many, many years without a problem. I bought mine directly from them when they were first starting up, before they started advertising.
There's no easy fix as the internal electronics have gone kapute. Before pitching the unit for a new one, contact Sonicare and see if they will be willing to work with you by replacing it for a discount.

If not, the Oral-B is FAR superior product and is the only type I see recommended by dentists. Really made a difference over the Sonicare.

Both are highly rated however. Ask your dentist for his opinion.

2018 reiview:
Best Electric Toothbrush Reviews of 2018 Reviews com