When you say turn off the valve what do you mean? If you are only using the hot water the valve should not be humming. If the programmer really is off, the boiler should not come on at all. (unless there is a frost stat wired in and calling for heat, very unlikely in this weather!)
Here is how a Y plan system operates with HW only:
From first switch on at mains isolator:
Valve motor will have no power and will be held at HW only by internal spring.
When HW is selected at programmer it puts power to the cyl stat common terminal.
If the water is not hot enough, the cyl stat sends power directly to the pump and boiler to heat the water.
When the cyl is hot enough to operate the cly stat, it's contact clicks over, so power is no longer going to the pump and boiler and they stop, and power is then sent to the grey wire (HW off) to the valve, though in this condition nothing happens, the valve stays where it is.
When the cyl cools down again the cyl stat clicks back and powers the pump and boiler once again.
That's it - end of. The valve should not move at all.
The valve will only move if the heating has been or is currently being used. If the heating was the last to shut off, the valve will start off in the heating only position, and will spring return to HW only, when hot water is next required. After this, it will stay in the HW only position forever, until heating is next used.
The test:
Turn HW on at programmer.
Turn cly stat up until it clicks, boiler and pump should start.
Turn cly stat down until it clicks, boiler and pump should stop.
Valve should not be humming at all and should be in the HW only position.
My goodness I don't half waffle on.