freezer doesn't stay cold---sizzling noise

I have a Philco No-Frost side-by-side refrigerator/freezer bought in 1970.
The freezer does not stay cold enough. I am using a freezer alarm that goes off when the temperature gets above 15 deg F: http://www.leevalley.com/garden/page.asp?SID=&page163&category=2%2C2120%2C33279&ccurrency=2 I can't get the freezer to stay below 15 degrees for more than a few hours. I wrapped the alarm sensor in foam and put it at the bottom of the freezer, consistent with the instructions.
With the freezer door open, sometimes I notice a boiling or sizzling sound just like the sound of water dripping into a hot skillet or onto a hot stove. Is it supposed to make that noise?
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Matt wrote:

http://www.leevalley.com/garden/page.asp?SID=&page163&category=2%2C2120%2C33279&ccurrency=2

Make sure fan and defroster heater is working normal. Tony
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Tony Hwang wrote:

The freezer fan turns sometimes anyway.
I don't know how to check the defroster heater. The freezer does not have any frost buildup. Does that mean the heater works?
Matt
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wrote:

If it is really that old, quit wasting time and electricity on it. Even a new 10-year-old unit will likely use much less energy.
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