My electric hot water heater makes a gentle hissing sound whenever it is heating water. There is no problem with the temperature/pressure relief valve. The sound seems to be coming from near the top of the tank.

If I turn cold or hot water on and off, the hissing sort of pulsates afterwards, every second or so until it slowly dies out back to a steady hiss in 30 seconds or so

I taught college chemistry, and had a distilled water still in my lab. The sound I hear from the HWH is exactly the one I associate with water touching the top of a partially submerged heating element in the lab still.

Is there an air pocket at the top of an electric HWH?

Is part of the heating element in an electric HWH exposed to the air in this hypothetical air pocket?

Is there an air pocket at the top of an electric HWH?
No there is no air in a water heater. Either the T&P valve is releasing a small amount of water or a leak is developing the tank.

As I'm sure you know water expands when heated and pressure increases. Because all hot water faucets are closed that pressure cannot be released through the hot water side. If there is no check valve on the cold water side, which there normally would not be, that pressure would move back up the cold water pipes and back to the street. However, if you have a PRV (Pressure Reducing Valve) that will act as a check valve and prevent the pressure moving back up the cold water side.

The problem could be that the T&P is failing and a small amount of water is being release as the pressure increases. Or there could be a small amount of sediment trapped in the seat of the T&P prevent it from seating properly. We don't usually recommend people manually open the T&P because as it sits there over time doing its job, mineral deposits can build up and then prevent it from resealing properly.

Of course the T&P could be doing its job and releasing pressure as it reaches its critical point. If you have a pressure reducing valve, it could be failing and the cold water in could be excessive. The cold water in pressure could increase because the valve is failing or because the city has increased the water pressure from the street. The output pressure is relative to the input pressure.

Manually open the T&P valve for about 30 seconds to flush the valve and hope that it reseals properly. If that does not relieve the problem get a hose bib type pressure gauge. About $10. See what your water pressure is when the water heater is not heating. Then monitor the pressure while the water is heating. If the pressure is below the blow off pressure, about 150 lbs then the T&P is probably defective .

You could have similar symptoms if you have a small leak in the tank that is developing. One that is so small that it only leaks when the pressure rises above the normal water pressure. However, if that were the problem I don't think you would get the pulsating sound after you temporarily release the pressure. The pulsating makes me think of a spring and that in the T&P.

If water pressure is normal, about 60 lbs or less, does not rise to the blow off pressure, and replacing the T&P does not resolve the issue, get ready for a new water heater, it's a tank leak.

It may be as simple as the fact that the surface temp of the heating element causes the water to boil at the element surface. I notice that with mine when I turn it on after being off for a while.

No there is no air in a water heater. Either the T&P valve is releasing a small amount of water or a leak is developing the tank.

As I'm sure you know water expands when heated and pressure increases. Because all hot water faucets are closed that pressure cannot be released thru the hot water side. If there is no check valve on the cold water side, which there normally would not be, that pressure would move back up the cold water pipes and back to the street. However, if you have a PRV (Pressure Reducing Valve) that will act as a check valve and prevent the pressure moving back up the cold water side.

The problem could be that the T&P is failing and a small amount of water is being release as the pressure increases. Or there could be a small amount of sediment trapped in the seat of the T&P prevent it from seating properly. We don't usually recommend people manually open the T&P because as it sits there over time doing its job, mineral deposits can build up and then prevent it from resealing properly.

Of course the T&P could be doing its job and releasing pressure as it reaches its critical point. If you have a pressure reducing valve, it could be failing and the cold water in could be excessive. The cold water in pressure could increase because the valve is failing or because the city has increased the water pressure from the street. The output pressure is relative to the input pressure.

Manually open the T&P valve for about 30 seconds to flush the valve and hope that it reseals properly. If that does not relieve the problem get a hose bib type pressure gauge. About $10. See what your water pressure is when the water heater is not heating. Then monitor the pressure while the water is heating. If the pressure is below the blow off pressure, about 150 lbs then the T&P is probably defective .

You could have similar symptoms if you have a small leak in the tank that is developing. One that is so small that it only leaks when the pressure rises above the normal water pressure. However, if that were the problem I don't think you would get the pulsating sound after you temporarily release the pressure. The pulsating makes me think of a spring and that in the T&P.

If water pressure is normal, about 60 lbs or less, does not rise to the blow off pressure, and replacing the T&P does not resolve the issue, get ready for a new water heater, its a tank leak.

Thanks for the quick reply...

The T&P valve on my electric HWH vents right at the tank; a drain tube (clear plastic tubing) just runs down the side of the tank and ends a few inches above the floor. There is no evidence of any water coming out of this tube.

Some of the diagrams for HWHs I've found on Google Images show the hot water outlet extending a few inches down from the top of the tank, possibly creating an air pocket, so I thought I'd ask...

Check water pressure. Remove access panel to to upper heating element and look for signs of water. A small amount of water could be being absorbed by the insulation.