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The hissing noise is subnucleate boiling (SNB) occuring on the element. A “long life” element is about twice as long as the standard cheap element and has much less SNB occurring so hissing is minimal. You might have a lower long life element and a standard one up top.
Several ways to check the lower element:
Ammeter – check current 4500 watts at 240 volts draws 18.75 amps.
With nothing else on in the house the wheel in the utility meter (with a k (sub-h) of 7.2) will turn at 5.76 seconds/revolution with a 4500 watt load.
With only a 4500 watt load a maximum of 13 minutes and 20 seconds may be required to advance one kWh on a utility meter.
Be careful with this one: With power off to the WH and top tstat satisfied, disconnect one wire from the lower element and float in the air. Turn power back on. Being careful to touch only the insulated portion of the wire, touch the end of the wire to the screw from which you lifted the wire then break contact with the screw. If you are drawing proper current you will get a good blue spark. Turn power off to WH and reland wire.